"I don't know much about art, but I know what I like"
Our current strategy is to have high quality art throughout all publications, full colour front covers and greyscale interiors. I get nervous sometimes with this decision, seeing people stating they are surprised the publication X used only black and white interiors in this day and age. However after looking at a few recent full colour publications it seems the case that only the very wealthiest and best selling publishers use great colour work inside throughout.
Adventure modules are for a publisher a little like the maps I used to pour hours into, colouring and drawing individual flag stones on, only for none of the players to see them. As DMs should see the adventures before anyone else, it is them that the interior art should be aimed firmly at. The cover, seen by all, entices them in with a good quality image suggesting some emotional key, as robin_d_laws rightly calls them, to kick start their imagination and convincing them to buy the product (or demand their DM runs it, if a player). The interior art though is far more integral to the adventure itself. While DMs should never, at least in a normal game, see the NPCs as 'their NPCs' they do need to empathize and get in the mind set of the opponents they lay out before the players. This allows them to roleplay the creatures far better as well as giving them a better idea of their suite of abilities and how they might go about using them. A good image always helps covey this, but a bad image does a great deal of damage towards this end. A goblin shaman can be a figure of amusement, being a hallucinating gibbering opponent who is as dangerous to his own tribe as the players or he could be a terrifyingly death cult midget who attacks with stealth and intelligence. Either way a badly draw, luridly coloured stick man destroys either imagery, consigning the shaman to being just another gobbo.
I don't want Raiders Guild DMs to fall out of love with their tools. Colour interior art adds cost to publications in three ways, art budget, printing budget and more difficult to quantify, time. I'd rather we had great pictures that convey opponents and situations that the DM can colour with their words than have them present lifeless stats that haven't invigorated them. Of coarse if we become one of the very wealthiest and best selling publishers, I'll have budget to colour them in for them too.
Next up, I need to think about players guides. Players often want more immediate gratification.