I have a Macintosh computer - I'm going to search a few sites and see if I can find one for Mac's.
Hi, SadLady. Why are you sad?
You have everything you need if you have an iMac.
Just to elaborate on Paul's post, when you are in iPhoto, click on the picture you want to post and go to "File," then "Export." You'll get a prompt asking what format you want and whether you want to resize.
Choose JPG for format and "scale images ..." then type in the size you want. Just type in one dimension and the other will adjust proportionately.
On the right side of that box, I always have checked "use filename" and "use extension" but you can choose whatever works for you here.
Then click the "export" button. You will get another box directing you to give your picture a name and a place where you want to save it. You can save it in a folder, or just on your desktop, wherever you think it will be easier for you to manage. If it's something I'm just going to post here and then get rid of the copy I've made, I usually just choose desktop.
Remember that iPhoto saves all your originals so you are free then to trash or do whatever else you like to the copy of your image that you prepared after you've posted it here, emailed it to a friend, or whatever.
I hope that helps. I'm sorry if you knew all this already! Let me know if you have any more questions.