Microsoft, which (boo hiss) owns Skype, has started putting ads on the Skype conversation screen.
Fortunately, there is a way to block them (at least, for the moment).
This is from the Skype community web site:
1. Open-up the Internet Explorer control panel (even if you don't normally use IE).
2. Open-up "Network & Internet" then "Internet Options".
3. Go to the "Security" tab.
4. Click on the "Restricted Sites" icon, then the "Sites" button below it.
5. Type "https://apps.skype.com/" (without the quotes) into the text box and click "Add"
6. Close and apply the changes.
7. Restart Skype.
I'm sure they will be back, but its nice not to have them for a bit....