Jumping off from this discussion, I'd like to hear more about what ISIS is all about, and whether I should consider using it. I work primarily with Cassini ISS data, and my main issue is that, since I work with rings, I'm interested in a cylindrical coordinate system rather than a spherical one. My other issue is that I'm accustomed to using IDL rather than C++, but I could consider changing if other issues came together.
Not all images are the same. Frequently, to do science you might want to compare two different images at the same resolution, or map them to the same grid, different from the original. To map to lower resolution you should co-add pixels, but to map at a higher resolution you might need to interpolate. This article discusses four interpolations schemes, how to use them, and which might be best for you.