Your use sounds to me like "fair use" of copyrighted material, which you can read about at this site: http://libguides.mit.edu/usingimages
you can also use this checklist to help you think it through:
https://www.lib.umn.edu/copyright/fairthoughts
According to them and their interpretation of US Copyright law, there are four factors to consider:
Nonprofit, educational, scholarly or research use; Transformative use: repurposing, recontextualizing, creating a new purpose or meaning
Published, fact-based content
Using only the amount needed for a given purpose; Using small or less significant amounts
If there would be no effect, or it is not possible to obtain permission to use the work
In order to make it more "fair", they suggest:
- Use lower resolution or thumbnail versions where possible;
- Place the image in a new context or use it for a new purpose; and
- Use only the parts of the image needed for the purpose