Try this and let us know if it works for you:
-Tap an image on your Collage canvas to select it
-Tap the Edit icon in the upper right corner
This puts you into mask mode, where you can create and edit masks for your image. I find it easiest to use the paintbrush tool, and "paint in" the parts of the image I want to keep (these areas appear at full opacity in masking mode); use the eraser tool to "erase out" parts of the image you do not want to keep (these areas appear at partial opacity with a checkerboard pattern in masking mode). You can also use the scissor tool to draw polygonal-shaped masks.
Here's a snippet of video that shows masking in action (I set this link to jump right to the masking part, though you may find the entire video helpful):
Let us know if this is helpful. Thanks for posting!