What are long tail keywords? Well, to start with you need to understand the nature of microstock and long tail theory in general. "The Long Tail," is a term that was coined by tech writer Chris Anderson in his article "The Long Tail" The piece describes how content providers - which is what a microstock agency like Dreamstime is - succeed not just by having "hits"; i.e. selling a few photos lots of times (bestsellers), but also but also by selling a lot of different photos just a few times.
As a content provider offers more and more stuff, it draws in more and more "niche" customers. Customers who are looking for that unique and unusual image.
So what does this have to do with keywording? Customers who are searching for unusual and out of the ordinary images are very specific in their search terms and search keywords (it's true - there have been studies!). They use narrower, rather than broader terms. They string words together. Even when searching for a general term like "family" or "law" or "love", they will pair it with one or more other words.
Keep this in mind when you upload your images. Sure your wonderful image is about "love" and "life" and "happiness" - include those words if you must. But to make your image stand out from the millions of other images with the same concept and keywords, ask yourself "what is specific and unique about my image?" - and then include that in your keywords.
Also try tunneling down into your keywords to find the narrowest term possible that is still accurate (as I described in my earlier post: "Concepts: One Thing Leads to Another"
Also pay attention to meaningful compound terms and be sure to include all of the relevant elements. Lastly, don't clutter up or stuff your keyword list with too many general terms - it doesn't work. It just doesn't.