Let me jump in here as this thread that just came back up. There is an important distinction between a tag and a keyword and understanding that distinction will help you when you are trying to get Google to notice your images.
On this site there is a section called keywords that you see when you upload or edit. It would be better to think of this as the tag section. This is the traditional spot to put in words that you want to match with what people are searching for. Those words are keywords. However in internet searches the" keywords" a person enters are matched with words found in the description and title. Internet search engines mostly ignore the tag section.
So it is important to put the "keywords" in the tag section to be found on FAA, but also, at the very least, in the description for your image, used in a sentence in order be indexed correctly in the search engines. This is basic to all web sites. Any discussions of keywords off this site is referring to words used in sentences and elsewhere. Tags is usually a list of search words and phrases to be matched with an onsite search.
So to recap this, a "keyword" can be used anywhere and refers to the words that you want to match an internet search. A "tag" is a keyword used in a list in a specific section to be used by a website to index images on that site and is largely ignored by Google and other internet search engines, as it is not a good indication of relevancy to the internet search.