[Plugin NextGEN Gallery] How to change Thumbnail file permissions

I am having an issue with the NextGEN Gallery plugin. I am running on Debian Squeeze with Apache. I am running version 1.9.12 of the NextGEN plugin.
Every thumbnail that is created in the gallery/cache subdirectory has 600 file permissions which means that the files are not readable by the www-data user. Manually changing the permissions to 644 solves the problem but is obviously not very convenient.
Is there a setting to change the file permissions for the thumbnails that are created by the gallery plugin? I have the impression that this changed with an update but I can not say which update broke the thumbnails.

Thank you,

Thomas