They are a few reasons that this could happen. The most common are:
1. You're attempting to use a .bmp or .tif or other non-supported file format. You can only use .gif and .jpg on the web. You must convert files that are not .gif or .jpg into a .gif or .jpg with your image/graphics program.
2. You've forgotten to upload the graphic files. Double-Check.
3. You've incorrectly linked to the images. When you are starting out, try just using the file name in the <img> tag. If you have cat.jpg, use
4. Image file names are case-sensitive. If your file is called CaT.JpG, you cannot type cat.jpg, you must type CaT.JpG exactly in the src.
5. If all of the above fail, re-upload the image in BINARY mode. You may have accidentally uploaded the image in ASCII mode.