It's not a problem with the max length of a URL. I think that number is set to 1024 in the code. In any case you are not bumping into it. There was also a problem with REQUEST_FILENAME being able to support only 128 characters. I've
fixed that too, but in your case, once again, you are well under than limit. So that is also not the problem.
Is it possible the file in question has an ACL that makes it unreadable by IIS?
I recommend that you compare the ACLs for the images that work correctly, versus the images that don't work correctly.
I just tried this here and I can reproduce the problem if I remove IIS_IUSRS from the ACL for the image. If I restore that ACL entry, then the image is displayed properly again.
This is on Windows Vista (IIS7). The principal in your case will be different if you're on a different OS.