This error message is produced by the validator when one of the images in the EPUB doesn't have a valid address as the source. Images in EPUBs cannot point to a directory on your computer or be linked from the Internet. They always need to actually be in the EPUB.
An example of a valid location:
<div><img alt="" src="../Images/001.jpg" /></div>
In this case, the image is located in the "OEBPS/Images" folder of the EPUB. If the image is in another folder, then that path should be entered.