The number one culprit for a file not showing up on a website is its filename. A filename should only consist of alphanumeric characters (numbers or letters), dashes or underscores. Any other type of character, depending on how the image is being displayed, could cause it not to show up correctly. If your image has any foreign characters in it, rename it, and give it another try.
It's also possible that, depending on how the images are being processed, the file size is too large. Make sure that the image is optimized for the web, and give it another try.
If you have tried both these options and the image still isn't showing up, please submit a HelpDesk ticket and one of our developers will get to the bottom of the issue for you.