This issue was happening to me all of a sudden. Happened just after uploading a 3MB photo via the admin. Tried messing with permissions and no help. Tried deleting the photo via FTP and suddenly was able to login again. Uploaded another photo and got the "appears to be forged" warning. Then it occurred to me that possibly I had maxed out my allocated storage. This was the case. I increased my space on my hosting account and everything is good again.