Keywords

One of the most important things that you can do outside of creating amazing illustrations is to use great keywords. Great keywords make all of the difference as to whether your illustrations get the views they need in order to make the sales you're looking for. It's how the people who can use your image find your work among the millions of others.
Say you have an image of a golden retriever. You have uploaded it and it is now available to anyone who can find it. Melissa is a designer and is creating a website for someone. The site she's working on is all about the golden retriever breed and even though neither of you know it, your illustration is perfect for the main image on the home page of the site. She goes to her favorite Microstock website where you have uploaded your illustration and types in golden retriever. Thousands of images load on the page. The site allows her to narrow her search to just illustrations which is what she wants to use. There are still thousands to choose from. Forty Eight pages of illustrations are available related to her search words and somewhere in the last 5 pages sits your illustration because you forgot to add the keyword golden. Chances are that Melissa is not going to look over the entire 48 pages of images to find yours. It's more likely she will go through the first five to fifteen pages and choose one of the illustrations that she thinks works for her. That's one sale lost. How many have been lost before or after Melissa, you'll never know.
Keep in mind that most designers are under a great deal of pressure and have very tight deadlines. When they are searching for an image or illustration to use in their work they aren't going to spend an excessive amount of time looking. The better your keywords the more you can help them quickly find what they are looking for.

How many keywords do I use?

If you are really doing your homework and have done a good job of choosing your keywords you should be able to narrow the number down to somewhere between 10 and 20. I use at least 10 because one of the website I upload my illustrations to has a minimum of 10 keywords that you have to upload. I don't use more than 20 because I believe more that that is not necessary and possibly irrelevant.

The Key to Keywords

The key is to make them relevant to your illustration and at the same time try to anticipate the words that someone might use when looking for images like yours.
Use accurate descriptions when possible. Instead of just including the word ribbon try using pink ribbon. Some sites will let you use compound keywords or keyword terms and some won't.

For Example

Here is an example of how you might choose keywords for the specific illustration below.

First choose the obvious descriptive single words. These are the words that represent what comes to mind when first look at the illustration
Mug, Beer, Green, Green Beer, Shamrock, Clover, St. Patrick's Day
Next are the words that are still relevant but not what you might first think of.
Holiday, Pint, Pint Glass, Drink, Froth, Illustration, Refreshment, Draft, draught, Lager, Alcohol
That's it. Those are the only keywords I used for this one. I had a lot more to choose from when I was picking the words out for the image but these are the only ones that I felt were relevant.

Here is another example.

Once again I started with about thirty words and narrowed them down to just these 12.
Tools, Rake, Shovel, Shears, Clippers, Prune, Gardening, Landscaping, Cut, Dig, Trim, Sharp
One point that I would like to make here is that you really don't need a lot of keywords but you do need good ones.
Some people will use as many keywords as the website will allow them regardless of whether they are applicable to their illustration. Some people will consult a thesaurus and use every word that is a synonym for the words they have already chosen.
I don't think that's the best approach. I live on both sides as a contributor and as a consumer of other peoples Microstock vector art and photographs. When I'm searching for something I usually have a good idea of what I'm looking for. Very seldom would I purchase an image or illustration that pops up in a search that is totally unrelated to what I was looking for.

If you have artwork of a specific geographical location you should include the location in your keywords. For instance if your illustration is of a beach scene located in Outer Banks, NC make sure you include that because someone may be looking for that exact thing. If the place has nicknames try to include that too. Some agencies won't let you include the keyword unless it recognizes it as an actual word, phrase, name, etc.
There are several online tools that have been created to help people to select their keywords. You can find a list of several of these in the resources section of this website. I find them to be helpful at times, especially if you have a very simple image and are having a hard time figuring out keywords that don't easily come to mind.