If you're using XP or Vista--after loading the pics onto your computer, got to My Pictures. Click on a picture to open the file. At the top of the new window, there should be an "Edit Picture" button/tab. Click it, then click on "Compress Picture" option on right. Click the radio button for "Document"-sized files--that size works pretty well here.
After compressing, don't forget to "Save". This will take a multi-MB file down to 200-500 KB, that is well under the limit for posting most places.