If you are set up in some capacity on your MOSS 2007 site as having Full Control, this won't necessarily give you access to upload and mange files in the Master Page Gallery. If you have been granted Full Control, but don't see all the options in the Master Page Gallery (see below), you also need to have the Design right. You can set this for the Group you are in or for at the user level.
Master Page Gallery options with and without the Design right. The lower screenshot is with the Design right.