Z2, JFI..I use a couple of different programs when I am doing...

Z2,

JFI..

I use a couple of different programs when I am doing batch converts for artwork..

http://www.faststone.org/
and
Quick Photo Resizer

As usual screen size used to be 640*480 so you could comfortably work out how much of the screen you want to use by dividing the screen up and transposing this to the above number..

The of course taking into account the DPI of the picture.
Which you can comfortably use 75-100dpi as a web-based jpg to post.

I also use
http://tinypic.com/
http://imageshack.us/
As they will auto size pictures to a good resolution for use on threads on the net..

Just a little share of info if you didn't already know
Cheers..