make a digital image file of art I like

What resolution/size do we need the images to be to be effective for different purposes? I have a rough idea about printing, but am not sure about how big they would need to be for making digital slides. Any clue?

I want to start building a database of digital images from gallery web sites, stills I shoot in galleries or from books and magazines, and work of friends and colleagues. Then I’ll have those images available to print out and look at for inspiration, make into digital slides, or incorporate into articles and papers.

I think this will be better than casting around frantically at the last minute, and much easier to deal with than making slides of a gagillion things when I might not need them all. I find slides really hard to keep organized.