As a digital printer, I prefer high resolution TIF, rather than JPG, but such files are massive in comparison. While some solutions are getting better, I've never found a consistently good quality image enlarger, not in 20+ years in the graphic arts industry.

It won't help your photo issue, but for mapping, the reason I use a vector application (Xara Photo & Graphic Designer 9) as opposed to an image editor (Photoshop, GIMP, etc) is that I create maps in units (usually inches) and not pixels. Pixels aren't even a consideration until I export the file, once I complete the map. While maintaining the same dimensions that I created the map, I am able to export to most graphic formats at any resolution I want and export an infinite number of versions in whatever resolution needed. And as far as what a vector map looks like, most people think I'm using Photoshop or GIMP and not a vector application at all.