This may seem like a simple one, but it caused a bit of head scratching, so it seems worth a post.
I was trying to use some JPG images in Content Editor Web Parts on a SharePoint site, so I put them into the Site Collection Images Library, which worked fine. I could see them there, and they had thumbnails, etc.
However, when I added them to the Content Editor Web Part, I got broken images. It turns out that the JPGs were CMYK rather than RGB. A designer friend of mine, Natalie Pangaro, was able to see this, convert the JPGs to RGB for me, and voila, everything was fine!