There are a lot of wrasse that do this (the most noted is the cleaner wrasse). Some fish come to expect it from the wrasse family, I see my angel swim up to a canary wrasse and lie on it's side holding it's fin up.. occasionally the wrasse will help out (not ich, sometimes just general irritants).. There are more suited fish to this task, however some fish like the cleaner wrasse, can't survive without this food source and are known to be reluctant to change, and should not be purchased.
They will pick at ich, but they aren't known to "cure it" and they themselves are susceptible to ich.. not good practice.
There are also gobies noted for this such as the neon goby that are also cleaner fish.
And as for the one in the picture.. there are bigger ones out there, and in some countries, they are food. reverse the picture..