@Biddygirl: Indeed. Simply put the inefficient was discarded (the painting was removed) to become more efficient (the text would be easier to read). Just what would Dilbert himself would have done, and Scott has done.
The art missing from the last frame was likely done because the words Dilbert says are laying right where the art would be hanging. Have the words over the painting would probably have made the frame too messy, crowded or difficult to read.
Dilbert is trapped in the Engineer's Mind Set. He is unable to actually releate to another human being on any other level than that of an engineer. Over his history, it seems that Scott has had Dilbert function in any number of different engineering roles. So now Dilbert usually sees things from a point of view of cost optimization. As itegem has correctly pointed out (as has Scott on several hunderd occasions) we live in a sub-optimized world. Everything is inefficient. So, everything needs to be optimized. So, to sum up - the painting is gone because Dilbert offended "art".