Pixlr from Autodesk gives you multiple levels of photo editing goodness, depending on your skill level and your creativity
I dipped into Pixlr Express and selected Collage:
You can find the grid patter you like, add photos, re-size and bada bing you got something going on that looks kinda okay for something I conjured up in 10 minutes:
Anyway, you get the drift, you can use existing photos or extract images from your videos put them together in a format that best represents your channel page.
If you want to go deeper you can use the advanced mode similar to most photo editing applications. If you want to make a collage but you need more assistance I'd look at the one at PhotoCat, you do the same things but there is a bit more structure and assistance.
One more post to summarize and then back to video.
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