Someone told me a good seo trick is to write additional keyword-rich copy beyond what your page needs and then hide that text behind an image. He said that search engines can't punish you because you can argue that you are merely describing the image (in more detail than alt tags allow) to Lynx users. Makes sense but I worry this sort of trickery would get me penalized. Does anyone here do this?

If you felt the need to ask this question you already know the answer.

or at least the long term results.

I probably shouldn't be telling you this, but if you're going to hide text, do it with JavaScript, because it is much harder to detect. That said, you should probably be focusing on things that will have a real impact, such as improving the site content, which will give you a much better performance/work ratio than any trick (which will likely eventually be caught anyway).

What you're talking about is a gray area. Sometimes I hide text to describe images that are images of text, when the designer or site owner is absolutely adamant that the images should be used for text, since they are "prettier". This usually involves using the image as a background of a heading (h1 for example), setting exact dimensions for that heading, then giving it a large negative text-indent so that the text is hidden and only the background shows. I think this is justified because it makes things easier for a screen-reader (used by blind people) to pick up.

your instincts are right! hiding is bad, it is deceit. deceit breeds (in you) a belief in unfairness. unfairness leads to failure. hiding, therefore, leads to failure. don't hide stuff. if you know it's wrong, don't do it. only do stuff you know is legit even if it is insulted and hated. then you can take the punches and know that it's only making you stronger.

(so what if a guy threw a shoe at me?)

Yeah I agree with those who say to avoid such tricks. Something like that may help a bit, but it is far too risky. A competitor could report the issue and your site could get banned. These days you usually can't get good rankings based on content alone anyway. Focus more on getting quality backlinks instead.

I'll never try to use blackhat techniques. And I strongly suggest you not to use those kind of tricks (blackhat). Maybe it will rocket boost your rankings in the first place but once you're caught.... you're gone.

White is still the best.

As with all forms of SE "trickery" the spam is not in the "method" but in the intent.

IF you are genuinely placing text behind an image for accessibility reasons, then that is a perfectly acceptable method. Put it there simply for SEs to read and you are spamming.
The intent of course will become obvious when the text is given a once over by the real humans in the SE "spam team".