If you find an image rates your thought and time to give critique do not minimize your gift by rendeing an apology for giving it. Phrases such as "Just a thought" suck the power out of your criticism. Be direct and unapologetic yet respectful. If you're going to say it then say it with conviction
Likewise if someone demonstrates how your image can be improved Be grateful! they thought enough of your work to show their idea on how it might be improved. be gracious and say "THANK YOU " then accept it or not!