use commas between key words.
spaces are optional, you can get more keywords in if you don't use spaces, though.
Don't spam by using keywords that do not describe the image. Don't be misleading.
Consider synonyms, scientific names and variety names, and a range of descriptive words from specific to general.
For example, if you shoot a waterfall in a park in Toccoa Georgia, consider:
Waterfall, Toccoa, Georgia, GA, (park name), cascade, water, cataract, stream, woods
if such words would describe the scene. Keywords may not show up or be utilized on Google and other search engines, so use the most important keywords in your description. Make sure your title words are in the keywords, also, generally.
Don't forget to describe important attributes of your image.
here is a checklist to broaden the types of keywords you might consider:
Literal - e.g. “boy, girl, children, beach, sun, sand, sky, car, shadow”.
Attributes - e.g. “happy, smiling, walking, blue, young, old”.
Location - be specific, e.g. “The White House, Washington DC, USA”; generic, e.g. “office, school, home”. specific to general: Location name, street name, neighborhood name, city, county, state, country, region
Interaction - e.g. “chasing, playing, moving, touching, shouting, flying”.
Concepts - e.g. “heat, power, happiness, strength, humor, teamwork, tranquility etc”.
Broad subjects - e.g. “lifestyle, travel, sport and business”.
Techniques - e.g. “predominant colors or shapes, black and white, grainy, blurred, digital manipulation”.
Media: oil, pastel, digital, sculptural
Standpoint - e.g. side view, rear view, from above, from below
Environment - e.g. spring, summer, autumn, fall, winter, sunset, sunrise, indoor, outdoor, interior, exterior
Composition - e.g. close up, macro, extreme close up, low angled, full length, cut-out, isolated
Minimalism - e.g. nobody, empty
Redundant - e.g. shots, pictures, images, macros
Pose - e.g. looking at camera, smiling, angry, standing, lying, sitting, walking, facing front
Human - e.g. one man, three women, crowd, queue, group, people, person
Qualifiers - e.g. large, big, small, multi-generational, black, white, caucasian
Abstract - e.g. odd, unusual, concept, quirky, funny, humor/humor
Decades - e.g. 1940s, 1950s, 1960s
Nationality - e.g. european, dutch, british, english, american
quantify: singulars, plurals
spelling: american, british, common misspellings, common foreign words (trunk/boot, wine/vino, quart/litre/liter, hippo/hippopotamus/hipo)