If you've removed that, it's a quick few clicks to get it back up and added to the site.  On the Admin tab under 'Site Structure', click 'Primary Navigation'.  There is an option on the right to add links to default sections (in this case, the Gallery).

The next step is to make sure that users can add pictures to the site - this is done on the 'Permissions' tab of Admin under 'Misc Permissions'.  Click the 'Allow' button for the Image Gallery and save the setting.  It should look like this:

There is a name field beneath your current galleries where you can enter the gallery name and click 'create' to get it on the site.

All your users have to do after you've done this is go to the gallery, scroll down to the bottom of the screen and enter the information for the picture they're uploading.