Can I photoshop pics of my girlfriend to make her skinnier?
That does not sound good. Are you implying she's fat?
She's a bit fatter than the media influences us to be. I want to Photoshop her to make her skinnier
I really don't think your girlfriend would appreciate you digitally manipulating her appearance. That's likely to make her feel self conscious.
It's fine. If a picture is worth a 1000 words and a picture can be replaced with a better one, why not go through the trouble.
Because it might upset your girlfriend. It will make her feel bad about herself that her own boyfriend thought that she did not look good enough and that he had to photoshop pictures of her.