When it comes to editting art (not just on Tumblr), the 2 most complained about points that you listed here are points 2 and 3. That is the main reason why many artists use a Creative Commons License. People, who think editting artwork sends the message that the original art was not sufficient as it, are too sensitive anyway. Maybe the artwork wasn't sufficient as it is. If that's the case, then editting the art work should be taken as a critique of some sort.