I've made a website that analyze keywords that ShutterStock customers potentially use in their searches:
http://www.stockkeyword.com/

FEATURES:
-This is a free online tool.
-It analyzes ShutterStock search results, as it's the biggest microstock site. These keyword trends are mostly similar on other stock sites.
-More than 3000 keywords already analyzed. Currently I add few hundred new keywords daily.
-The site provides data for the first 100 search results for every keyword.
-all images/photos/illustrations/vectors  and  popular/relevant searches
-The data shows competition factors and potential sale opportunities for each keyword.
-Each keyword page also shows the most important related keywords for every keyword. This way, it's easy to find the best keywords for your images.
-You can find new keyword ideas, that have great "estimated sale factor" and/or big "low competition factor".
-You can view all the keywords of the top 50 images on one page - it also helps proper keywording.
-Average and newest ID shows whether it's possible to get a new image into the first page for a given keyword, or it's reserved for ancient images. (low ID means old image)
-Related keywords are clickable, the site is easy-to-use.

If You're interested in a given keyword(s), reply to this post, and I'll add it/them in a few days!

Hmmm......this looks like it should be good but it all seems rather complicated. I looked up 'cat' but can't begin to understand the significance of the numbers.  Maybe you can explain?
average ID top10:    171 057 228
average ID top100:    156 319 257
newest  ID top10:    302 691 488 (10.pos)
newest  ID top100:    302 691 488 (10.pos)

ID-s are growing linearly, small ID means old picture, big ID means new picture:
200 000 000 --> submitted approx. 2014.06.20
300 000 000 --> submitted approx. 2015.07.15
(My first image was submitted to ShutterStock on 2014.01.09, it has an ID of 170 490 818 - it was never bought  )

average ID top 100 means that the average submit time of the top 100 images was 2 years ago.
newest  ID top100 that the newest image in the top 100 is about 3 months old

So, if the newest ID is below 200 000 000, it means that it's pretty hard to get a new image into the top 100.

The site use computes a formula - called "low competition factor". The bigger is this number, the easier to get into top 100 for that keyword.

Alone, this number tells nothing, but compare with the numbers of other keywords to find out, which keywords has lower competition.
Here are few examples of "low competition factor":
cat: 134.56
kitten: 429.60
kiwi bird: 66704.75  (much less buyers search for this word, than for cat)
linx: 1560526.74 (it's a misspelled version of lynx - lynx we'll be online on today's update)
background: 0.84   (it's almost impossible to get into top 100)

estimated sale factor can be more important


The other usage of the site is to get keywords for your image. Little downward in the section called:
Top related keywords for CAT (images, popular search)
You can find the most important relevant keywords in order of importance. For example:
KEYWORD    IMPORTANCE    NUMBER    IMPORTANCE/NUMBER
cat    9472    97    97.65
cute    4126    75    55.01
pet    3772    79    47.75
kitten    3596    68    52.88
animal    3213    76    42.28
white    2884    46    62.70
isolated    2162    47    46.00
vector    2033    23    88.39
mammal    1828    41    44.59
dog    1752    19    92.21
kitty    1743    49    35.57
feline    1717    56    30.66
portrait    1652    39    42.36
black    1625    29    56.03
fur    1513    35    43.23
background    1410    41    34.39
breed    1198    24    49.92
paw    1157    25    46.28
domestic    1058    68    15.56
young    1047    42    24.93
love    1038    21    49.43
funny    985    37    26.62
adorable    937    38    24.66

Add them except of the few irrevelant, then click on "kitten", for example.
Then add new keywords like: baby, small, sweet, etc...