Constructive Feedback
“Morale boosting” statements are fine.
Constructive criticism statements are fine.
However, the poster needs to make it clear what they want to hear from members;
An all comments welcome or stating nothing at all indicates they are open (fair assumption). If the poster specifically says constructive criticism only, then those who feel they can’t provide advice to assist the poster should not make comment. Simply stating “great image” or “you’re stars are too blue” is not good enough and only introduces “noise” into the thread. Tell them how you would go about addressing the problem – work with them publicly so that others can glean knowledge from the thread. There are a lot of knowledgeable people here with numerous levels of astrophotography experience – leverage off it by being specific for what you would like to hear.
For those that feel inferior when posting a sub perfect image, remember people have traveled the same road before you. You will only improve with experience and knowledge sharing. Get out there and post those images and ask for constructive feedback. Don’t take your images to heart. Acknowledge them for what they are and look for flaws.
When I stop learning, there will be a medical report classifying me as officially deceased.
Image Editing
What’s the point of editing someone else’s image from this site?

As it is, you're limited to 150k file upload size, thus the uploaded file is small and highly compressed. If you seriously think you can improve the uploaded image while it is such a compressed state, I wish you the very best of luck.
Try it for yourself. Open the image that you've posted and modify the curves or colour balance - I doubt you'll improve the image as the compression has reduced how much data you can work with. It will introduce more noise and have a "washed out" look. The only way to improve an image is to have access to the raw data. So lacking this - don't bother. Just provide constructive feedback if you have the capacity to do so.
If the astrophotographer provides a link to an unprocessed higher resolution file(s) specifically asking for assistance or to simply have a go at processing it, then you can make a difference. In such a case, you should provide information as to the exact steps they have used to process the image. Also, those who reprocess the image should also acknowledge that the data still remains the property of the astrophotographer. Don't try and rip it off and post your version on your website (I have seen this happen) or use their data for layering within your own images without consent.
I find the 150k limit restrictive, but understand there needs to be a limit. Those in the know usually provide links to their external website to better convey their work so that don't have to comply to such file size limitations. After all there is no point spending all this time collection data and image processing into a masterpiece when you can't display it in all its glory.