
Real easy... 4 steps to take:
1.you make yourself a custom tileset. (best to use idraw)
that tileset has to be in the default color of the tile (mostly pink or green or so...) so in fact you have to have a blanc tileset in one default color.
2.then in the rm2k(3) maker you simply go to part where you create a new tileset, select the one you just made and then decide which tileyou want to be passable and which one needs to block (so only two tiles are needed) -> if you don't know what i'm talking about on using tilesets, read the manual in the rm2k(3) program itself... that's how most ppl learn it...
3.make a new map an choose a background like the ones that lun made (I still am in awe about the 3d graphics and the way he compresses them without losing any of the quality

, unlike what i'm capable of doing ... wish he could explain how to do so... maybe i'll bug him bout it later ...

)
4.you use the new tileset you made on this new map. use the button that makes your whole map to go to the same tile (autofill) and make it all impossible to go trough anywhere... then simply take the brush and clear a path wherever you want to walk using the tile that doesn't block...
and presto, you're walking on a picture background...
If nothing of this seems to be working... then i'm sorry, I just wrote this tut in 2 minutes... go to gamingworld to find some decent tutorials there... i'm sure u'll find what you need in there and more... (btw i'm not to explain the use of idraw... that's trial and error... figure that out urself

)
cya