Sounds like a problem I had once when migrating..
This CAN happen if you migrate using a shell command like wget
To test if it is that problem:
1. check one image file.. back up the thumb, image and original on your harddisk.. delete these three on server and then restore them with ftp client from your harddisk..
2. test resizing and rotating with THIS image
If it now works.. you have the same problem I had..
If it still doesnt work.. then its another problem..
What happened to me:
I commandline gzipped the site and fetched it on the other server..
the problem is.. i was logged in as root.. so all the files were owned by root now.. and all permissions got set to "owner root"
Only new files were working...
I couldnt even chmod the image files with my ftp client: Error-> no permission..
I could only delete the files with ftp client.
When copying the same file AFTER deleting it.. it worked..
So the First time i fixed the problem by making a backup of file directory with FTP client on my local harddisk... then deleting the directory on server.. and then restoring with ftp client..
Now ownership of files was no longer root but "nobody" and it worked..
Happened to me a second time.. i found a tool to change ownership of files (but cant remember what its called
THIS DOES NOT HAPPEN IF YOU MIGRATE USING STANDARD FTP CLIENT to transfer the files. If you used standard ftp client.. then its another problem..