Hi Milou,
Perhaps the following articles will help demystify map projections and how they are used:
http://en.wikipedia.org/wiki/Map_projection
http://www.colorado.edu/g...pproj/mapproj_f.html
What is the map information for each image and what kind of images do you have i.e. Landsat, SPOT, Ikonos...? The other question is what do you plan to do with those images ?