I do not think this is a question that has a straightforward yes or no answer.
The CC NC clause is really hard to get a grasp on, and Creative Commons do not provide much guidance about it. Their FAQ: Does my use violate the NonCommercial clause of the licenses is deliberately vague, and can be summed up in this sentence:
Whether a use is commercial will depend on the specifics of the situation and the intentions of the user.
The FAQ answer about the NC clause concludes:
CC cannot advise you on what is and is not commercial use. If you are unsure, you should either contact the rights holder for clarification, or search for works that permit commercial uses.
The NC clause is controversial, and there has been repeated requests to discontinue it, such as this one from the Free Culture Foundation.
In 2008-9 Creative Commons commissioned a study from a professional market research company that resulted in a report with the promising title "Defining Noncommercial", but this report only summarizes how Internet users perceive the terms “commercial use” and “noncommercial use” in the context of content found online. It does not provide a legal definition of these terms.
The study was met with some hostile responses. Here is the one from free culture proponent Glyn Moody:
All that comes across to me from these figures is that "non-commercial" is so fluid a concept as to be useless.
I do not agree with Moody. The wily NC-clause fills a much requested niche in the sharing (but not free) culture ecosystem - for instance it helps supplying schools and NGOs with materials that otherwise in all probability would have been ARR. But the NC has a very narrow use case. It is for creators who want their creations to stay free (as in free beer), and not being exploited for money.
But since no relevant case law exists (AFAIK), and the concept is so difficult to pin down, it is not prudent to use materials with this clause in borderline cases (and I think said freemium app is a borderline case), unless you are really keen to contribute to case law by being sued over borderline use.
I do not think it is possible to give any defintive answer to your question without examining the specific app and the specific business model in detail - and even then the answer may turn out to be: "Definitely maybe".
But we can not provide this analysis for you on this web-site. If you really need to use CC-BY-NC-SA images in your app, you must to hire a lawyer and have him/her access to your app and documents describing your business model and give you professional legal advice about your specific case.
But since you're asking here, it can be presumed that you're unsure about this, so the best answer I can provide is to repeat the answer from the CC FAQ:
If you are unsure, you should either contact the rights holder for clarification, or search for works that permit commercial uses.
