Just that. Show an image in sound description, so a visitor can see what's playing.
Already there: http://freesound.org/people/qubodup/sounds/60030/
Of course it's up to the sound owner to decide to upload, host and embed an image.
Hosting images outside of the web is not optimal. You can see that all over the early posts of various forums. 99% of external files are missing. And it is sad .
At least some kind of thumbnail (with enlargement on click) would be nice. If Soundcloud can have one... ?
Then there are all these problematic youtube reposts... there is a picture under play button.
Perhaps integrating picture gallery into the freesound player in the similar manner as you can view waveform and spectral plot via a click of the button would be the solution. Showing multiple picture thumbnails slidable from side to side...