Are you using images that are licensed under creative commons? You have to specifically search for those images and even then you have to double check.
If your using google image search, you need to click on Search tools, and then Usage Rights, and then check off "Labeled for reuse with modification."
Another search tool:
https://search.creativecommons.org/
I've used those 2 methods for my apps and haven't had a problem.
