"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
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.
Yes the pictures and art in WH40K can make the codex coffee table books, if you live a nerdhold ;)