At Dreamstime keywords are not weighted by importance. They all get equal weight. At the moment there is no way or need to order them.
They have been making great strides in eliminating spammed words lately. Who knows, maybe weighted keywords are something they would consider in the future...?
They seriously need to do something more technically sophisticated than just weighting the title words. The default search order at DT keeps bringing up tons of similar images (by the same contributor) on the first few pages of searches. Try a search on 'Pasta' for example and check out pages 2 & 3. Virtual 'image spamming' like that is annoying enough without it being rewarded with a prominent sort-order placement to the detriment of others. Now try the same search anywhere else and you'll find much more variety in the results.
I think IS and SS have by far the best system in which the actual words used to find/buy images are the primary factor governing where the image appears in the default sort order. Rather handily it should effectively eliminate keyword spamming over time by virtue of shoving all the irrelevant images way down the sort order on a given search.