Preserving paper stickers long term
The greatest culprit of the fading of printed colour is UV light. This process is called photo-degradation.  

You can protect your images by framing them using conservation glass or UV filtering Plexiglas (which can be found at most art supply stores and framing ateliers), or coat the glass you already have with UV protective spray. This process needs to be repeated, though. There are also (adhesive) protective sheets you can cut to size, either to cover the frame window or the individual objects, but these are harder to find.  

Apart from hanging your pictures out of direct sunlight, be sure to frame them cleanly, and place them in a stable and dry environment, away from heat sources.