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Take your time

First of all, when keywording an image, take your time! This is important, so you might want to do some thinking. Trust me, this will save you time in the future. Think about the subject of the image and how it may be used and choose some keywords that are most likely to be used by a person that might want that image. After deciding the main key words, look for synonyms (choose mostly common words, don’t add fancy ones that may not be used by most buyers). One way to find all the words that matter is to ask questions about the image and to answer them with a few keywords. Ask these questions: what?, who?, where? when? why? how many?. Ask yourself if the image has any other meaning (like a conceptual one). For example a photo of an ice climber can contain keywords like "risk" or "danger".  

Don’t misspell!

Many stock photographers are not native English speakers. If this is your case, make sure you do not misspell your words, or your images will be lost in the wind. There are many ways you can check you spelling: you may use your word editor’s build-in auto-correct option (for example with OpenOffice or Microsoft Office); you may consult a dictionary (check this one out: http://dictionary.reference.com/), etc.

Keep in mind that there are words that are written in more ways than one in English language (American English versus British English, for example). I will give you just a few examples: trousers (British) – pants (American), football (British) – soccer (American), colour (British) – color (American), and so on (for more information about this subject visit this page on Wikipedia). In this case you should both forms of the word.

How many keywords?

This depends on the conditions of your stock agency. Most have an upper limit that may be 30, 50, 70 or more keywords. I believe that, in most cases, 50 words is a reasonable limit and I hardly use more. If your agency has no limit number of words, use as many as possible but keep in mind that some agency will reject images with inappropriate keywords and others penalize the photographer for keyword spamming. So do not abuse. Fewer words are sometimes better, if they are good. I often find images with less that 10 keywords ranking first in searches.

Keywords you should not use

Some stock agencies may have some restrictive rules regarding the use of some particular words and they may even close your account if you do not respect these rules. For example, the use of the words “teen” and “nude” (“sexy”, etc.) together is not recommended if the model is not of legal age. For more information read your agency’s guidelines.

Using names

Use country, town, city or place names ONLY if the photograph is representative for that location. If you have an image of a pigeon taken in Paris don’t use the word “Paris” as one of your keywords. Of course, if the photo of the pigeon has the Eiffel Tower visible in the background, you can use the keyword “Paris”. If you image features as the main subject a certain plant or animal, you should add as keywords the scientific name for it. Buyers looking for that plant or animal, are familiarized with names of species, and they will probably search for the scientific name.   

Plural or singular?

Most stock photography sites have search functionality that insures that an image will come up in a search both for the singular and the plural form of a  certain word. You should always check that this is the case for the agency that sells your images. If this doesn't apply then you should use the word in both forms, just to be sure your image will be found.   

Embed keywords into the photo

There are many software options that allow you to assign keywords, description, title, etc. to an image. I like to use Adobe Photoshop (since I use it anyway for other stuff). In Photoshop, open your image, go to File>File Info and insert the data you want into the appropriate fields (this is called IPTC file information). You should always have a title, a description and keywords for any given image. This is usually the information requested by most stock agencies. Title, description and keywords are automatically extracted when you upload your image to the website of (most) stock agencies. You will find this useful if you upload images to many stock sites, since you only have to keyword an image once and then the keywords are automatically filled in every time you send that image to a stock agency. With Adobe Photoshop, you can also export keywords from a file and then import them to another image, to save time. Do not keyword JPEG images in Photoshop, because every time you save a JPEG file, the image is compressed and artifacts may occur. Save the file as JPEG only after you finish all the editing, before uploading the image.  

Other tips

Use keywords like “exterior” or “interior” for architectural images, this will help photo researchers.

Keyword batches of photos if they fall in the same category (copy-paste keywords from one image to other similar images). Of course, you should remove any inappropriate word and replace it with other one if necessary.

Do a little research: visit the website of any stock agency you want and try finding an image similar to the one that you have, by using their search engine and the keywords of your choice. This is how image buyers look for a photo, and this will help you test the relevancy of the words you are planning to use.

If you have images with a calm, empty area, suitable for droping someting in (like some text), use keywords like "copyspace", "copy", "space". Designers are always looking for this type of images and words like these will buyers find them.