Image in signature

Hi, I created an html signature in Thunderbird about 2 months ago and, up until this morning, it was working just fine. Now, when I save or send an email, I get this message (I use Thunderbird un French so this is a translation): "A problem occurred during the insertion of a98h.png in the message file. Do you want to continue sending the message without the file?" The strange thing is that there are 3 images in my signature and only 2 seem to be causing a problem. All 3 are hosted on Imageshack. I even tried re-uploading them and using new urls, but I get the same problem. Do you know why I have this problem? Thanks,