I have been using this site, and find it very useful.
They are using thumbnails in a search in the same way as Google and other search engines. I don't object to seeing my images used in this way.
Keywords are not top secret. I have looked at other peoples keywords in order to get inspiration. I tend to keyword my images, then look at this site to see if there is anything obvious that I have missed, for example the word 'elephant' in a picture of an elephant. (Yes I did miss that!)
To me, keyword spamming is using words that do not relate to the image in order to drive traffic. For example, using 'sexy woman' on an image of a sofa. This site seems to me to be promoting the use of appropriate keywords rather than spamming.