Hi Dave! As you can see I've hijacked your post to explain the process. It took me a while to sort it out myself so I've got pictures!
First go to the sub folder under skin discussion that best describes your skin. Notice that there is a test folder as well for works in progress.
Dr. Drrae did a sample layout
Once you are in the folder you want pick New Topic and you'll get a "Post a Reply" screen.
First upload your screen shot. Scroll down till you see the Upload Attachment section. Click Browse to find it on your system and then Click Add the file once it's found.
You'll see a progress bar (or part of it) while the file uploads.
Once that's done there will be a new section at the bottom of the text box named Attachments. Click on the reply screen where you want the attachment and then click "Place inline".
It will then look like this:
Type in the descriptive info and then repeat the process for the *.skn or zip file for the skin. This system doesn't seem to have a way to link to a URL for attachments.
I hope this makes sense,