Sorry to bring up an old discussion, but I am having the exact same issue on one of my sites now, and I can't seem to find much info about this in Google.
Some hints point to setting headers in htaccess or php, but I have tried that and it is not resolving the issue.
I don't think yummyftp is causing this, I'm just hoping for some insights in this issue.
The strange thing is, that sometimes when I rename the image on the server through yummyftp the error goes away. I'm not sure if that is significant, but I can't seem to find any info about this...
Any help is appreciated