Is this legal? I mean is a free application always considered
NonCommercial in terms of CC BY-NC-ND?
Without pretending to legal expertise which I do not possess, the best answer I can give is maybe. Note that the definition of NonCommercial is defined as follows:
NonCommercial means not primarily intended for or directed towards commercial advantage or monetary compensation.
From that, I would interpret that if the primary purpose of the app is to make money for (or otherwise gain some commercial advantage for) the creator e.g. through serving ads or through the use of in-app purchases, this wouldn't be compliant with the NonCommercial clause.
Potentially - again, I stress I am not a lawyer - it sounds like if the app has some commercial element (e.g. a donate button) but the primary purpose is not to make money, this may well be a legitimate use of an NC-licensed image.
As to the second point - is a free app always considered NC? - the answer there appears to be that this wouldn't always be the case. At least in principle, I can conceive of a free app which nonetheless primarily serves a commercial purpose e.g. although the app itself is free, it exists primarily to promote a paid service provided by a particular company. This would seem to meet the definition of "commercial advantage".
I would note that Creative Commons themselves are fairly vague on the exact definitions of commercial vs. non-commercial use. From the Executive Summary to Defining “Noncommercial”: A Study of How the Online Population Understands “Noncommercial Use”:
Perceptions of the many use cases studied suggest that with the exception of uses that earn users money or involve advertising – at least until specific case scenarios are presented that disrupt those generalized views of commerciality – there is more uncertainty than clarity around whether specific uses of online content are commercial or noncommercial.
At the end of the day, I suspect these definitions are vague enough that it would have to come down to legal proceedings between the creator of the image and the app developer before a definitive answer could be found as to whether this specific case is legal or not.
TL;DR - It might be legal, but it probably depends on the specifics of the case. Ask a professional IP lawyer.