Create a 64x64 pixel image of the person in question. Save it in your Pictures folder (or wherever), so you have easy access to it. Name your image file using the following convention: name@company.com.tiff (it is important to use the extension .tiff, not .tif). Drag your chosen images onto the relevant address cards in Address Book. In your home folder, go to Library. In the Library folder, create a new folder called Images. In the Images folder, create a new folder called People.
Copy your 64x64 tiff files to this new People folder. Mail should now see the images, provided they have been named exactly as the e-mail addresses in your address book (with the extension of .tiff at the end). If you have friends with more than one e-mail address, duplicate their image file and rename it with the alternative e-mail address using the naming convention above.
[robg adds: This works as described, and I hadn't realized how much I'd been missing the images in Mail. You'll have to quit and restart Mail to see the new pictures, though, as it seems to cache them at startup.]

