I am assuming that you uploaded the graemlins via FTP.
If you did then, the size of the images will be stored incorrectly within the database.
You have two choices (AFAIK)
1) Edit the database manually to set the correct size for each image.
2) FTP the original images up (to correct the fault) and then add additional graemlins via the upload system built into the control panel.
Personally I would do option 1 - it might take a while but at least they will all be correct. You will of course have to reverse the method when you FTP the original images after Christmas.
I do not know of any other option - sorry.
If there is one then, I am sure someone will add a comment