For older submissions I think that the best solution is to cancel the old upload and to resubmit the picture

I also do long photo descriptions for SEO reasons, for some of my photos, such as these photos with at least 300 words descriptions:

http://www.jareso.com/photo/73/water-sawmill-in-kvacianska-valley-slovakia/
http://www.jareso.com/photo/69/budatin-castle-in-zilina-slovakia/
http://www.jareso.com/photo/131/southern-side-of-famous-orava-castle-slovakia/
http://www.jareso.com/photo/79/broadcasting-transmitter-against-the-blue-sky/
http://www.jareso.com/photo/573/close-up-of-two-pigeons-having-bath-in-a-fountain/

They seem to pick a lot of traffic from SERP, when compared to photos that have descriptions under 50 words. My goal is to have 300 words descriptions on all of my photos, but it goes very slowly; because such descriptions take a lot of time to be written.

You can read more about it at my blog:
http://www.jareso.com/blog/how-to-rank-better-in-serp-with-personal-portfolio-website/

Along with the long photo descriptions, translations also play a very import role when it comes to ranking and picking of traffic from SERP. Translated versions of my personal portfolio website actually pick more traffic from local - language specific searches - than those in English.

Again, read more at my blog, if you want:
http://www.jareso.com/blog/one-little-secret-about-those-much-wanted-stock-image-buyers/

Search engines would decompose long sentences into individual words. it is is like have more keywords added in different location. If in your description words from keywords list are re-used it probably does not help.

I thought that many agencies provide localized version of image pages and some even localize keywords. Suggestion to translate your page to different languages make sense only for self hosting.