Technically selling photos isn't 'outside work' in my opinion. It is selling personal property.
Copywrite basically states that any work you create is the property of the creator unless given up with permission.
This means these photos are your things. Selling your football, bicycle or other possession does not breach contract or visa laws. Photography is a hobby. You are taking photos in your free time and taking them for yourself, even if you hope to sell them afterwards. You are not employed by an agency, so you aren't technically working.
I personally think that if anyone even raised an eyebrow about it, you easily have enough loopholes that it wouldn't even be an issue. I mean if they even consider it work, which they won't, it isn't work that is taking place on your Japanese visa since you are not employed in the "outside work" within Japan.
That's my 2 cents. But really, don't worry about it. Just take Word's advice. No taking and of course selling pictures of students.
Oh, and congrats on being able to sell some of your prints! That's great! Although I'm a bit unsure about Getty.
