in your admin control panel what is set under the
Conversion tool for thumbnails?
Mine is set for GD ...
not sure if that is where the problem is or not...
update...
look for this in the download.php file
$multi_var = multi_download_resize($multi_resize,$multi_new_image,90,$download_multi_sizes[$multi_download_select][1],$download_multi_sizes[$multi_download_select][2])
look for that ,90, in that line. I was using that for compression. I think in the GD library it gives you 90% of the quality.
maybe one of the others are thinking you are wanting to compress it by 90%.
Test it by changing it to ,10, for 10% compression and see what it does...
You will have to delete the resized files so it will re-create them again for you...