It's been a while since I uploaded to dreamstime, but from what I recall you can put in a category and then click to copy that category over to the next photo - it will give you the option to copy categories, and also keywords, etc. from around the last 4 or 5 photos you've uploaded.
All the photos you upload there are "unfinished" until you check the categories, and make sure your caption and keywords are correct. It also adds all the words from your caption to the keywords, which is really annoying and useless if you write your captions as proper sentences, so it will take you some time to go through - but after correcting one, you'll be able to at least copy them over to similar images.