I'm on an iMac. Are your photos within iPhoto?
Within iPhoto:
This is what I do as my photos are in iPhoto.
CLICK: photos I want to post
CLICK: email icon located at the lower right corner. This will open up a window asking you what size you want the photos. I usually keep mine at "medium".
MAC MAIL: your photos will now appear within an email window. Drag these photos to your desktop (they are now resized).
Return back to the message board--post your topic/photos.
Try this and it should work.
The other way to resize your photos is to look at your camera settings. There is a setting for quality of photos. Sometimes there is an email setting on your camera. Change this setting to something that isn't the highest quality. That too will help to resize your photos.