What this tutorial will cover:
- What is GIMP?
- How to install GIMP?
- Getting started.
- Enhancing images.
- Modify the colors of an image.
- Applying a pattern background to an image.
 What is GIMP?
GIMP is a free, open source and cross-platform image editor.
Image editor: As the name implies, it does pretty much whatever you want to do with an image: Change colors, crop and paste, layers, effects etc. It is equivalent to Photoshop in capabilities.
Free, open source and cross-platform: That means it is free to use, free to modify and works on all common operating systems: Windows, Mac and Linux.
 How to install GIMP?
 Just click on this link, and follow installation instructions.
 Edit (June 29, 2018): This tutorial was written for GIMP version 2.8.22. Currently a newer version is out with changes that can make this tutorial hard to follow. If you want to follow the same steps as in the tutorial, it is possible with this link (for Windows – Choose the option ‘gimp-2.8.22-setup.exe’).
 Getting started.
For this tutorial there are 3 windows you will need to know about:
- Main Window
- The Toolbox Window
- The Tool Options Window
When you open GIMP for the first time, you will see the following window, which is the Main Window:
Image 1: Main Window
For this tutorial you will need a Toolbox. There are many ways to open a toolbox, here is one of them:
In the top menu bar, click on tools. In the menu that will open, click on ‘New toolbox’.
Image 2: Opening the Toolbox
A new toolbox will open. It should look like this:
Image 3: The Toolbox
Now we will open a new Tool Options window. Double-click on the bucket icon. The Tool Options will open.
Image 4: The Bucket Icon (right) and the Tool Options (left)
 Enhancing images
Let’s say I have an image that is 700×400 pixels, and I want it to have the dimensions of 1500×700 pixels. This is something that will cost in resolution regardless of how it is done. Here is a suggested way to do it so that the loss in resolution is minimal. This is the most common use of GIMP for DDG, from my personal experience.
First you will need to open the image in GIMP. There are several ways to do that, here is one (tested on Windows operating system):
Right click on the image, then select ‘Open with’, and choose GIMP.
Image 5: Loading image into GIMP
Once the image loads, select ‘Image’ in the top menu bar, then select the option ‘Scale Image’.
Image 6: Selecting Scale Image
The Scale Image Window will open.
Image 7: Scale Image
In this example, we will scale the image to the dimensions of 1500×700 pixels. This we are changing the ratio of the height / width, we will need to unlock the ratio functionality, by clicking the small chain icon.
If you clicked correctly, you will see the icon changing to an open chain icon. Now enter in text field of Width and Height, the values 1500 and 700.
Image 8: Unlocking ratio functionality
(Optional) For best results, I recommend changing the Interpolation from ‘Cubic’ to ‘Sinc’ using the dropdown.
Image 9: Changing Interpolation to Sinc (Optional)
Finally, click on ‘Scale’ to perform the scale.
(Optional) Now there is another technique that helps make a scaled image seem less ‘blurry’ after loosing resolution.
In the top menu bar select Filters, then Enhance, then Sharpen.
Image 10: Opening Sharpen filter (Optional)
For best results, I recommend playing with values between 80 and 95. It is best not to pass the 95.
Image 11: Sharpen filter (Optional)
Finally, you will need to save the changes. For that there are two relevant options: Overwrite and Export.
In the top menu bar click File, then on either ‘Overwrite…’ to overwrite on the same image, or Export, to save as a new file.
Image 12: Overwrite or Export
In case of Export: A new window ‘Export Image’ will open. In ‘Name’ set the name of the image, but do not remove the file type. If you want to save as a different file type change the file type accordingly (e.g. ‘new image.png’ or ‘new image.jpg’).
Image 13: Export Window, Setting new file name
Finally, click on the location you want the file to be saved to, if it is different from default (Under ‘Places’), and click ‘Export’.
NOTICE: In some cases, it asks you after Export what quality would you like to save the image. Unless you have a specific reason not to, you can safely set the quality to 100%.
 Modifying the colors of an image
In the top menu bar, under ‘Colors’ you can find all the tools for modifying colors. For all practical reasons you will most likely only use one of the top four:
- Color Balance (Will be covered in this tutorial in more detail).
- Hue-Saturation (Allows changing the Hue, Saturation and Lightness of the image, and even of a color).
- Colorize (set one color for all the image. This option is likely to be less commonly used).
- Brightness-Contrast (As the name implies, set brightness and contrast of the image, since this is simpler it will not be covered)
Image 14: Modifying colors – Useful tools
In this tutorial I will only go over the Color Balance option, with a specific example.
If you click on ‘Color Balance’ this window will open:
Image 15: Color Balance
The Range to Adjust (Shadows, Midtones and Highlights) basically means the dark, medium and light colors. Each can be modified separately.
You can play around with the colors, and only press ‘OK’ when you want the changes to apply to the images.
As an example, here is what happens if I set the Midtones to 100% Blue:
Image 16: Color Balance – Before and After
Finally, after you get the changes you want, click ‘OK’ to apply them to the image, and overwrite / export.
 Applying a pattern background to an image.
Let’s say we have an image and a pattern we want to apply at the background of the image. We will be working for this example with the following images:
Image 16: The pattern image we want as background
Image 17: The image we want to apply the background to
First open both images in GIMP. Do that by right click on first image, open with GIMP, and then do the same for the other.
Image 18: Both images open in GIMP
Now select the window with the background pattern, and copy the image to clipboard using keyboard shortcut Ctrl + C.
With the pattern image in clipboard, open a new toolbox for the image you would like the style applied to (right image, the tomato, in this example). To learn how to open a toolbox follow the ‘Getting Started’ section of this tutorial. Click on the same Bucket icon from Getting Started to open the Tool Options for that tool. Now click on the fill option ‘Pattern fill’ and click on the pattern icon. Select the option ‘Clipboard’.
Image 19: Setting Pattern fill to Clipboard
NOTICE: The Pattern icon should change the pattern you copied.
Now, click on the background to apply the background pattern.
(Optional) You can scroll down in the Tool Options till you see the ‘Threshold’. By playing with different threshold values, you can fill different spaces more effectively.
Image 20: The Pattern fill result and the Threshold option