Okay... how to give constructive criticism:
Saying something is "bad," "great," "beautiful," or "unrealistic" is not helpful because the artist can't do anything with that kind of vague feedback. The artist has been given no advice on how to stop being "bad" and thus and will reject the opinion as uninformed and unhelpful. Remember, it doesn’t matter what you believe. It only matters what you can prove.
For close analysis, the more concrete your observations, the more persuasive they become. Try to avoid imprecise words like “interesting,” “bad,” “unrealistic,” “beautiful,” and the like. Such empty words tend to raise more questions than they answer. Try being more specific. Have several reasons why your reader should be persuaded to agree with your argument. Have two or three pieces of evidence to support each point. The more specific your evidence to your argument, the more persuasive it becomes.
For example: What is the impact of drawing our interest in this manner? How does this observation advance your argument? Why did you find this art not useful as a DM? How does this art expand the written world of the module? Does it conflict with the text in some manner? Does it reveal things about the depicted that you would like to remain hidden at the time that you would use it?
So, to use myself as an example, I wrote that while I admired much of the art work presented in Pathfinder, I wondered sarcastically what a stiff wind would do to Seoni's street reputation. This was an unpersuasive statement for me to make because it immediately setup an antagonistic relationship with the artist without providing a reason for why I was being snarky.
A better way of putting that concern would be to state that I found Seoni's pic to be too influenced by pin-up illustration, that it was somewhat demeaning to have only women drawn in pinup style and that it would make my introducing this module to my girlfriend more difficult. Had I written this instead, it would have given the artist an understanding of my concerns as a consumer and as a person promoting the product.
So, to meet my concerns, he might decide in the future to illustrate women in a more restrained manner.
Or he might have more male meat shields depicted sweaty and with their shirts off!