I save all of my images at 300 dpi in BMP format. 300 dpi is high enough resolution that I can rotate the image without getting artifacts, and I can see extremely fine detail. BMP is a "lossless" format that saves the raw pixel data without any compression. An image can be edited and saved multiple times with no degradation.

Images I share on the web are 150 dpi and in JPG format. JPG is the most universal compression format. But it's "lossy" - that is you lose detail. If you edit a JPG image and resave it, you lose more.

With my masters in BMP, I can create a JPG at any resolution at any time. The BMPs take about a Meg apiece, but with a 100 Gig hard drive and writable CD, I don't worry too much about size. If you want lassless compression, use ZIP.