This article is from the eBay FAQ, by William B. Shaw firstname.lastname@example.org with numerous contributions by others.
If the links you provided to your pictures don't match the actual URL of the
picture, your pictures will appear broken. Also, some free webhosts block
links from eBay's servers and don't allow picture hosting from their sites.
Try cutting the URL for your picture and pasting it directly into the
address bar of your browser. If the pictures still don't work, you'll get an
error message stating why. If they suddenly work, your web host is probably
blocking the link from eBay's site. You'll either need to upgrade to a paid
service or switch to a different web host.
EBay provides some additional help here.