There are free softwares available to help you resize pictures to fit on the screen. I use a program called photoimpression but it's cabailities are pretty limited.
To post pictures you need to have your files stored online at a place like photobucket or your own website. Then just do this: [*IMG]http address[/IMG*]
Otherwise, GIMP (I keep Portable GIMP on my USB Flash drive) is a great free one and you can find other commercial software packages on the Internet if you know where to look.
Once you have them the way you want for posting here (go 800 pixels at the widest, IMO), host them somewhere to link to, srothfuss suggests.