The problem is, that iPhoto has a build-in export feature that lets you resize the image, but not change the jpeg compression.
Quickest way: Drag image from iPhoto onto any photo-editing app in your dock. I use Photoshop (commercial) or GraphicConverter (shareware). Resize the image to 640x480 or so, then save the jpeg with a 75% comperession level. You should have under 100kb jpegs doing this.