Desmume (and many other emulators on all systems really) will have something like a OAM viewer, VRAM viewer, Map viewer and whatever in the tools menu (of Windows at least). You might have to select the appropriate palette.
If the graphics are loaded into RAM (either because it is on screen or soon could be) then you can snatch it right from it.
Harder to find hidden graphics, or rare graphics (if a sprite only appears in after a 40 hour sidequest...) but this is one of the reasons we have cheats
-- infinite health, tricking conditions, if you can do a character select cheat then that...
So yeah you can get them without background just by playing and having access to the mighty print screen button. Way less of a barrier to entry than conventional ROM hacking might be where you have to figure out formats, possibly compression and more besides (though many will often snatch palettes from RAM to feed to other things if they are not obvious) but also a bit more limited.