On sol 708 (August 3, 2014), Curiosity performed a series of short drives followed by MAHLI imaging of each of the wheels, to survey their condition. Here, the images have been sorted and the inter-cleat spaces numbered to make it easier to survey the locations of specific marks, tears, and punctures.
Here is a closer look at the damage on the left-middle wheel over time:
NASA / JPL / MAHLI / Emily Lakdawalla
Damage to Curiosity's left-middle wheel, sols 546, 660, and 708
Inter-tread spaces are numbered the same from image to image to help orient you to the locations of holes in the thin metal skin separating the grousers or treads. As of sol 708, the left-middle wheel is the one that has suffered the most damage.