The usergroup that is allowed to upload images needs to have create permission for the galleries they are allowed to upload images in. So at component level (Backend > Components > RSGallery2 > Control Panel > Options (toolbarbutton)), "Create" permission would be inherited, effectively "Not allowed", this way you user cannot create a new gallery at the top level. Then for the gallery where they are allowed to upload images (Backend > Components > RSGallery2 > Gallery > [Edit the gallery] > Set Permissions), set the Create permission to Allowed so that after saving the 'Calculated Setting' is 'Allowed'.
With this setting you users can still create subgalleries in that gallery, however, for the frontend My Galleries you have to option to limit the number of galleries users can have via Backend > Components > RSGallery2 > Control Panel > Configuration > tab: My galleries > Setting "Maximum number of galleries a user can have" = 1. This way from the frontend a user cannot create any more galleries if they already own one gallery. From the backend these users could create more galleries though.
Perhaps the new feature mentioned here http://www.rsgallery2.nl/nightly_builds_rsgallery2_joomla_22.214.171.124/what_will_be_new_next_version_posting_guidelines_download_16673.msg43927.html#msg43927
is something you might already want to use. It adds a "Create Own" permission. The difference between "Create" and "Create OWn" is analogues to Joomla's core permissions "Edit" and "Edit Own". "Create Own" (allowing the user to upload images only in galleries they own) is currently only implemented in the Frontend, which is also the reason that there is no official release with this.
If you want to use the "Create Own" that you'll want to make sure that the gallery that you create for a specific user has that user as its owner when you create it: Backend > Components > RSGallery2 > Gallery > [Edit the gallery] > Set "Owner" to the user you want.
What do you think of this?