Just sounds like it's being scanned in an uncompressed ("raw") format. Open it up in preview, do a Save As, and change to some lossy format like JPG.
Then close the file, and open the JPG. Since the new format won't have a specified DPI anymore, you need to make sure it's not HUUUUGE resolution on your screen. Preview will try to open it to full size, but if it's bigger than the screen (probably is) it will zoom it down. Hit cmd-0 or select Actual Size under the View menu.
If that blows up too big on your screen, go to Tools, Adjust Size, and then scale it down. Preview will helpfully tell you how big your proposed new size is, so pick something you like. Save it.
Now just attach the smaller file. But you might want to keep the original. Edits and crops and cuts etc are best done on the larger file, to conserve detail. If you may want that detail later, keep the original.
PROTIP: open several images at once by selecting them in finder and dragging to preview. Get them showing up in a thumbnail list on the left, with the selected one on the right. Click over on the left to move the focus, then do a CMD-A to select all the thumbs. Now go to Adjust Size and resize them. Your adjustment will be done to all the images you have selected. If you changed quite a few, you may get to see it do its work one at a time when you try to close the window or quit preview. I use this when resizing images down taken from my digital camera when I don't need the huge files or massive resolution.
