Displaying posts with certain custom fields only

I have a bit of a random blog. Several posts have photos or other graphics I’ve made as a portfolio of sorts, but most of the posts don’t. I’d like to integrate thumbnail images on a random few of these posts on my front page as a sort of mini gallery alongside whatever my latest entries are. And I’d like each of those thumbnails to link to their full posting.

I can handle displaying the thumbnails and linking with the Custom Fields and an extra included php template I make, and I’m sure once I’ve got things all set, I can randomize it one way or another.

The problem is that not all my posts will have thumbnails, so I’m wondering if there’s a way to only select posts with that particular extra field. And I’m -positive- there’s got to be SOME way to do it. I just don’t know whether it’s already been done, and can’t seem to find the answer in the support forums here. Any ideas?

I’ve also thought about grouping all these posts into a category and displaying them that way, but I’m just wondering if there’s a better way. Any advice is appreciated. Thanks!

-Velda