If I go to images.google.com and type in a search term (for instance "clouds") I get a number of images returned as a result. But for a given image in the search results, how can I find all the different search terms that would return that image? For instance, a given picture might also be found with the search term "castle" or "grass", how can I find out if that is the case? Obviously it is prohibitive to try to search every single possible word and see what search results Google returns and try to correlated all the possible image results!
If it's publicly available, look at the site map for the site to which belongs the image. Also look at the webpage that contains the image.
Some keywords could be assigned due to the similarity of the image with other images with a higher relevance index.
Bear in mind that besides that Google likes to do experiments, their index is dynamic and the their algorithms are tuned and changed very frequently, so if you find an image that you like to find again, could be much reliable to save the URL by bookmarking or sharing it.