I'm the original Reddit thread poster. I did not intend for this situation to be posted on HN nor go out of hand. I will deal with the situation with the CEO privately, but for now I am chalking this up to a misunderstanding.
In the end, it was 100% my fault that I was not cautious. Please, no internet lynch mobs - there is absolutely no need for that.
If your only mission is to adjust one asset by 2 pixels, they're similar. If you want to see how those 2 pixels affect the design in its entirety, then photoshop is a much better option.
The best way I've found to learn design is to imitate, imitate, and imitate. Go on dribbble.com, find designs you like, and start emulating them on Photoshop. Eventually, you'll pick up on good design conventions.
If I were you, I would find a technical co-founder. If your girlfriend can't help with the creation of the startup, it's not a good idea to partner up with her.