If at all possible, I would advise making the illustrations resolution independent. By that, I mean making best use of (if you're using Photoshop) paths and related tools such that the final resolution of the images is not an issue whatsoever.

Illustrator, Photoshop's bigger brother in this regard, offers a wider range and more specialised tools for a job like this so it would be my recommendation to use that (or indeed, any other industry-standard software) to make the illustrations: from that base, any scale of artwork can be rendered for the web.