There are a couple settings that may need to be changed when uploading zip files via the NextGen Gallery admin interface within WordPress. The actual plugin notes that php.ini should have the following setting changed from the default of 2M to 20M like below.
The default location is typically in the /etc/ directory so use your favorite editor and modify /etc/php.ini.
Upload File Size Setting Configured In php.ini:
upload_max_filesize = 20M
However there is also another post max size that should be modified in php.ini as well like the below.
Maximum Post File Size Configured in php.ini:
post_max_size = 20M