Your best bet here would be a specialized RSS feed set up on your server. You could bundle a dashboard widget with your theme that automatically pulls this RSS feed and displays the thumbnails and a description of the new skins that are listed on your site.
Step 1: RSS Feed
First, decide what information you want to display in the widget. I recommend a skin title, the thumbnail, a short description, and a link to further information.
Store this information as XML on your server.
Step 2: Dashboard Widget
Create a dashboard widget that routinely checks this feed and displays updated items on the dashboard. You can use just about any existing RSS reader widget as a model here… updates to your server-hosted XML file will automatically appear on remote WordPress dashboards in this section.