Well, on my Craigslist entry form, there are four big boxes that appear (I think after you put in the text). There are BIG X's in each box. Click on each, remember where your pictures are in your computer (exact file path) and then double click on the one you want. Lather, rinse repeat 3 more times to get all your pics in.
Maybe it's 6 pictures, can't recall for sure, but hopefully you'll get the picture. Pun intended.
If not, try this:
The first part kind of explains the uploader tool, but the rest explains how to resize your pictures. It's really much easier than one might think. If I can do it, anyone can.