I've just written a guide on how to create a forum friendly image so as to comply with The Photo Rooms guidelines. You'll find that we use the same guides as most photography forums to as to make the site quick to load and accessible to those who don't have access to fast Internet links.
I'm a Windows freak so it's been written using Windows. I also use Photoshop CS2. You'll probably find that it's not very different in Photoshop Elements or CS3. There are lots of other tools out there, some being free, e.g.
http://www.faststone.org/FSResizerDetail.htm. I'm not even going to attempt to instruct anyone on all the available options. The instructions in this guide should be a good starting point to understand the concepts.
You can find the guide here:
http://www.thephotorooms.com/forum/index.php?action=tpmod;dl=item10. You can also browse to it in the Downloads section of the site under "Tutorials".