Don’t think that just because a picture, diagram, or other artwork is on the internet and easy to download that it is free to use because it’s in the “public domain.” If the image is copyrighted and you use it “commercially,” you could be subject to substantial statutory and other damages. There are numerous businesses scouring the internet for copyright infringement recovering substantial bounties from the infringers they find.
Be sure you can use the images you download before using them.