PS Express creates the fastest collages that I find satisfactory for deep dream; perfectionists may want to take the quick results into Photoshop or Photoshop Mix for iOS for advanced customization, which is made simple, so the limitations are less limiting.
First I choose “make collage” and select up to 9 images.
Next, I select an image grid for the layout. I can resize and reposition each individual layer to better fit in its frame, or change their positions by tapping and holding one, then dragging it to another.
One of the most important customizations to me is the ability to remove the margins between images, find that option via the border section under “radius”. If I want to, I can also resize elements of the photo grid by dragging the frame (or edge) between images.
When I’m happy with the composition, I can export it to my photos, or export layered versions for further editing into Photoshop CC or Photoshop Mix for iOS. If you want masking or other tweaks that go beyond the capabilities of the app, this is the way to go.