In the last few months my team and I at Austhink software have had a wonderful graphic designer -- David Urbinder -- consult on our products. Much more than design icons and images, collaborating with David has led to improvements in overall visual design, and more deeply into dynamic behavior (interaction design) and consequently the overall usability and appeal of our products.
Working with someone from a complementary design background helps to trigger creative sparks. David has pointed me to this article by Nate Fortin about the non-separability of design into different disciplines in Cooper Journal. Cooper Journal is hosted by Cooper Design, whose principal Alan "Father of Visual Basic" Cooper wrote the influential book About Face, and the provocative and important The Inmates are Running the Asylum, both of which I also highly recommend.
One of the issues that we are grappling with is how best to engage expert outside design consultants like David. At the moment we are doing a blend of some consulting time, plus directed tasks, spread out over time. The benefit of the spreading out is that we get the benefit of David's creative input early, when it can have a formative effect, and guidance at later stages too as the product becomes more "locked down". Coordinating our schedules is a "fun" part of this approach, and is of course more challenging than just employing him for confined blocks.
Finally, if you want to commission David, I would love to link to his web-site, but I believe that it is not ready for public consumption just yet (hint, hint ;-). In the meantime, David can be contacted on email at email@example.com.