Page Template & Post help

I noticed there aren’t any really good portfolio plugins out there. I like the custom ones that people have done that are of a large image with a small amount of info below but you click on it and read the full details.

Here is what I’m needing a little help on. I was to write a custom page called portfolio.php that is populated by a certain category. I want the image that shows to be specified by a special field in the post. Can anyone help me with this?