Unfortunately Constructs editor wasn't designed for this kind of grouping. Most users who need that kind of functionality have opted to create their own editors. If you do a search on the forum, you'll probably notice quite a few topics on the matter.
If none exist I guess building/editing your levels in photoshop is the best/only option.
Actually its not a very good option if you plan on importing large images into the program. Construct works best with many smaller (optimized IE p^2) images.
Its not very time effective, unless you use a custom editor, but on the other hand, smaller images used as multiple instances do not add to vram once the initial image is imported.