I came across a weird situation today - that I am surprised I have not encountered (or remember encountering) before. Images that show up fine in all browsers apart from IE8 - which displays the broken image with a red cross. Came across this by chance when we were doing some cross browser testing - and of course it was a rare inconsistent issue that affects IE...
The scenario: We have a site that displays user supplied images with colorbox showing the images in a popup.
The same images show in the page OK but don't show in the popup colorbox. Although the file url is correct and displays fine in all other browsers.
The Reason: The reason for this is that the particular user supplied image is a JPG saved in CMYK mode (not typical as most JPG saving programmes convert to RGB).
The images on the page were using a Drupal style (aka ImageCache presets) which converts the image to a smaller version and one that is RGB (as it happens)
The Colorbox popup was set to show the 'original' image and that is why it didn't display in IE8 although it shows up fine in all other browsers.
If you read up on the subject you'll see plenty of good advice about converting your images to RGB - but that is not helpful in the case of user supplied content.
The resolution: The answer therefore is always to use an image style when displaying an image in Drupal!
If you are not using Drupal then you will need to convert your images to RGB - The main issue here is how to find out consistently which images are RGB and which are CMYK without having to convert them all!