Designers sometimes overthink things, and I have a gut feeling this question is possibly an example.
One of your main roles in UX, most of the time at least, is to reduce users' performance load (cognitive and physical). Given the context you have provided, I fail to see how the choice between "pictures" and "images" can have an effect on users - seems to me both are fine.
Collins on "picture":
- A picture consists of lines and shapes which are drawn, painted, or printed on a surface and show a person, thing, or scene.
- A picture is a photograph.
- Television pictures are the scenes which you see on a television screen.
And these definitions are what I think of as a "picture".
An "image" is anything that hits your retina. So, subjectively, I would go for "images".
There are Always Surprises, Nearly
If you ever done user-testing after an expert evaluation, you know that there are nearly always surprises (experts say X, in user testing you see Y). So probably the best thing to do is a bit of user testing.
Gorilla testing is probably better than no testing. Sure, it is not statistically rigorous (most user testing experiments aren't anyhow), but sometimes the responses show a very strong trend.
- Create a lo-fi or hi-fi mockup of the app.
- Show it to people (street, friends, Mechanical Turk) you think fit the target audience.
- Do not use the word "picture" or "image" (or any related words), just use "these things".
- Ask: what would you call it?