Great news! Google Image search now includes a filter for searching images licensed for re-use! Just go to Google Images and search by keyword. Then under search tools choose the appropriate license under “usage rights”
For example, here’s a search for images of rainbows licensed for re-use with modification.
Not sure which license you need? The Creative Commons website explains how you can (and can’t) use work based on its license. Or, leave a comment below about how you want to use images, and I’ll help you figure out which license is right for you.