On a website for a client, I named one of the images Advertisement.jpg. I thought this made sense because it was a picture of an old advertisement. The problem was in one of my Firefox profiles I couldn’t view the image on the website. I banged my head against the wall (figuratively) trying to figure out why in this one case the image wasn’t showing up.
It turns out I was using AdBock Plus in the profile and it was blocking the image because one of the preset filters was configured to block files with advertisement in the name. After changing the file name the image suddenyl appeared.