Thumbnail for post in sidebar?

I’m trying to put thumbnails corresponding to the image/video in each post in the sidebar. The topic has come up before, and there’s a clear way to do it — but it requires you to add a custom thumbnail field to each post. The people using the blog are going to need something that generates a thumbnail automatically, or it will end in tears.

There are two plugins out there that are supposed to do this: Post Thumb Revisited and Post Thumbs. The trouble is, I can’t get either to work — though since I put the n00b in newbie it may be that I’m just clueless.

Does anyone have experience getting either of these plugins or an alternative to work?