How did you access this folder   /_layouts/images?
I need to go to the IIS manager and drill down to the default  website to finde the _layouts web etc.
Could it be that you are actually putting the image file  in a _layouts/images folder under your local Inetpub/wwwroot ?
Maybe that is why it can find the image to load on the page when you try it and not when others are trying it.