I recently had need of finding out how to make a collage of various painting images for the Palette Knife Painters group. It's not as easy as I thought but I finally found a site called Fotor.com which does a limited amount for free before you have to pay an annual or monthly fee. Paying annually is cheaper of course. And it's easy to use.
So here is the first collage of three of my own abstracts.
Sometimes it doesn't show the image in full - as in the lower two images of this collage - but you do have a few layout designs to choose from. Not perfect, but useful.