I know that backtrack 3.0, which is based on Slackware, uses a 'themed' console with an image background. The screenshot below just shows the bootscreen but after the system is finished booting the user logs in through the same themed interface. How would one accomplish this with slackware?
http://www.newbie.cl/nb/wp-content/u...backtrack2.jpg

That looks like bootsplash in verbose mode. You have to have a patched kernel for that. There's a forum thread here on how to do it from the last few days.

Perhaps I did not make myself clear, but I'm not looking for the bootscreen to be themed, but for the terminal itself (no graphical login). Is bootsplash capable of this?

Thats what bootsplash is -graphical login and graphical login with splash are two other separate things.
If you want something less intrusive that lets sou display images on a framebuffer terminal background, you might look into 'fbv'. It may be a little difficult to find working sources, but I have sources, patches, etc for it here:
http://distro.ibiblio.org/pub/linux/...s/fbv/fbv-1.0/

The picture you showed is without a doubt bootsplash, though. You can use that even without graphical login. It principle, it replaces the penguin logo you see at boot-time with a full-screen image. If you sue splash=silent in the bootparams, you won't see the text at all unless you press F2 during bootup. But, if you use splash=verbose you'll something just like what the picture shows -with the terminal messages showing in a box inside the screen. You can change the size of that box if you want, change the background image or other tricks. If you use 'silent' you can show a progress bar as well. You can even show animations if you like.

Thanks for the info, I will try bootsplash as soon as I get the chance to recompile the kernel.

Though not exactly what you are looking for, you may also want to check out bashish at http://bashish.sourceforge.net/