Phil has you covered here, Gabriele!

Abie

the caption is the second most read part of a post when skimming...so should you use it ?
i tend to add curiosity statements to see if i can stop hte skimmers to read that section at least....
the description is what shows up when you use the image as an attachment page (lightbox) - maybe another chance to add topical words....
definitely use the caption....
up to you for the description...