I think it would be very helpful if we could see images of the words in the vocabulary, such as for animals, numbers,colors, places or things.

I know it takes time to add these images, but using some search engines to collect the images and then crowdsourcing their meaning would greatly reduce the problem and serve as an added exercise.

Perhaps double clicking them or right clicking them, could send us off to some search engine to see the image.

Even sound, or short clips are known to improve memory retention. Indeed, studies have shown that images and multimedia content may improve foreign language acquisition (Carpenter & Olson, 2011; Kovacs, 2013).

A colleague of mine found in his research that games with visuals helps expand a vocabulary of a student. An alternative to developers adding images and media content could be a tool of some sort to help individual "duolingers" add/attach pictures (or media content) to words for just for own profile.

(Carpenter & Olson, 2011; http://psychology.iastate.edu/~shacarp/Carpenter_Olson_2011.pdf)
(Kovacs, 2013;http://groups.csail.mit.edu/uid/other-pubs/gkovacs-meng-thesis.pdf)

Edit: Rather than always having to hear the pronounciation of the word, why not just the sounds. For example:

"mooo" --> A cow ---> la vache "splash" --> water/liquid ---> l'eau "ding" ---> bell --> sino