How is one supposed to know the translation of a word that has not been defined yet?
[Further explanation: this specifically arises in the "give a word for this picture" exercises, where (i) there is no context, and (ii) there is no word to hover over.]
I think that part of the process of learning a new language is the ability to intuit meanings based upon what we already know in our new and mother tongues.
Alternatively, hold your mouse cursor over the word to get a list of possible translations, and pick the one that makes the most sense. : )
It's a problem in the program which happens unfortunately. For the picture clues, hovering doesn't work. I'm lucky, because I'm using memrise.com for vocab on the side I usually know what the translation is, but if you don't either guess, google the English hint word or take the hit on the heart and memorise it when it gives you the right answer.