There is an image gallery built into every Guild Launch site by default! There will be a link on the primary navigation bar (between the header and the content of the site) for the 'Gallery'.
Step One: If The Gallery Link Has Been Removed
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).
Once you've got that set, just 'Add Default Link' and you're set.
Step Two: Granting Upload Permissions
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:
When these two things are done, users will be able to upload images to galleries in the Image Gallery! Now you just need to create the gallery itself, which is done on the Gallery tab on the site.
Step Three: Creating A New Gallery
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.