If you're wanting something a bit more dynamic you could add the promos (so image and link) as Theme Settings. This would let you make quck edits without having to dig into the code. Of course, you'd need to add that code into the theme in the first place.
If you're curious about how to make new theme settings check this link:
If not theme settings you could also create special collections or linklists. The contents of either could also be used to help show a promo. So if the "promo" collection has an item show it in the sidebar. If it doesn't, show nothing. This too would need some theme edits.
|an hour ago|
|2 hours ago|