working w\ different digital image types

Hello all!

I would like some sites for information on the different types of digital images and how to work with them.

What is the best format for images you want to upload to the web?

How do you convert images that are large JPG images to something easily posted on the web or sent via email?

Are there any freeware programs that are available for doing this?

Digital photography tutorials would be great!

Thanks in advance!
Lizzy