I tend to use photobucket if I have lots of photos to post, since a) it eliminates the limit of three attachments for each post and b) it is a lot faster to upload and it has no limit to the size of the photo attachment (it does however restrict you to a 500 x 500 pixel display size on the forum).
Anyway - this is what I do to "shrink them" with standard Windows tools; my apologies if you have a Mac:
1. Open the picture in Photo Editor (it should open there by default)
2. Choose the selection box (icon with a dotted rectangle), and select the area of the photo you want by dragging a box across it
3. Right click and select "crop" - this will discard the rest of the photo. Steps 2 and 3 are not necessary if you don't want to crop the image, but they will retain resolution!
4. Under the "Image" menu, choose "Resize", then set the largest of the two pixel sizes to 500; this will ensure you can see if the resolution you'll be left with on screen is good enough for publishing
5. File/Save As with a new name
6. Load to photobucket and post (instructions on how to do this are under another thread in this section, I think)