'- if the author explicitely asked for critical comments, then I post also my critical point'

I agree with Orio. The only time you should post critical comments is if the artist specifically requests it. Otherwise, an encouraging note is all that's needed. Either that or just move on to another. I think we're all aware of the deficiencies of our renders - someone pointing them out is just redundant :)

Also, I think that in many cases comments are mostly just a checking in by friends to say "Yup - I noticed your post today" :)