Say we are creating an educational application that needs to use a number of images as part of its lessons. Since it would be prohibitively expensive if we had to hire an artist to create all the necessary images, would it be legal to use images from Wikipedia Commons in a product that we sell?
This is their creative commons license, but it doesn't mention commercial product use.
Yes. The introduction to the license says "You may adapt — ... build upon the material for any purpose, even commercially". (My emphasis).
Note that there is an attribution requirement so you need to find a way to attribute each image to its individual author.
For the fine details, you certainly need to read the actual license (rather than the introduction) and you probably need to consult your own, paid-for, IP lawyer - but if you are just scoping the project out at the moment, you should be fine.
For contrast, here is a Creative Commons licence that does prohibit commercial use.
