It could be many things. But the fact that you're getting a black area where the images should be tends to indicate that the images are missing. That could be because you forgot to upload them, or because you uploaded them to a different location than is specified in the code.
What often happens though is that there's a mismatch between the upper and lower case paths and/or filenames of the image files. Upper and lower case almost always matter on the web, almost never on the local machine. For example my.jpg and my.JPG are seen as the same locally, but on the web they're two different files.
But there could another reason or reasons. If you want more help:
Please post a link to a page on your site that contains the problematic code so we can check it out.