Though we did have a pretty nice (semi)automated feature to generate all unit icons from within the engine, in-game, with the actual in-game units, transparency, team color and so on... However this uniformity also came at a cost of recognition.
Now we have rules/guidelines for the build pictures, to still keep some uniformity, but also the freedom to make each and everyone unique and more recognizable.
We even made a custom map to support all the features, area's and background we needed to make the best of them.
So we now have unique pictures for ground, sea, seaplanes, air, amphibious, water, underwater, hover and even differences between tech levels.
As an extra bonus, we also found a way to really improve our overall UI sharpness. With new techniques we now have practically every graphic in the interface on par with the super sharp in-game visuals (due to the also recently upgraded Adaptive Sharpen feature)