Part of the pictures size is what resolution it is saved at. When you save them, pick a low resolution, like 72 dpi, and that can help.
Personally, I use photobucket to host my images. It's free, it's easy, and you dont have to worry about space limitations like you do with Reef Central's gallery.
What you do is make the account on photobucket, then upload your images and just link to them in your reef central post, they will show up in the thread, it's very easy.