The first step: You like a image!
The second step: You like it so much, that you will comment it!
The third step: You see, with "YOUR EYES", something to make better.
The fourth step: Ask you, if the artist has intended this, what you will criticise, or if this is a technical deficiency.
The fifth step: (if the last is pretended accurate) - make a neutral comment and write a IM to the artist, about your criticism and motivation.

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Possible opinion and criticism is very much individuel and complicated.
Criticism, constructive criticism, has to refer only to the engineering of a program, not the idea of a picture, if present or discernible.
To write a IM, is for sensitive artists less a way to compromise them!

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And a little notice too: What is a "wip"??? For what is a "wip"??? If a "artist" need a lot of opinions to make a final work, ..., so it is no more his own work, but only the work of many opinions...