I think all the extra activity you have on Dreamstime can result in more sales.
If you make a Blog and add some photos of yours, people will read it and see some of your photos. Maybe they really like it, are interested and love to snoop around in your portfolio. This can already result in a sale. Withouth the Blog, those people maybe would never have found your photos/portfolio.
Though I think that the Blog needs some good content and (maybe) needs to be helpfull for others. This way it will be more populair and more people will read it.
Next to this, the Blog will also be indexed by search engines. Which can result in people finding your Blog on Google or other search engines. This also can also result in some sales of course.
This doesn't count for blogging only. I think some activity on the forums now and then can also profit you in some more sales. As a webdesigner I also buy photos from Dreamstime. Most of the photos I bought, where from people that are active on the forums. I found some photos there I wasnt able to find with my keywords on Dreamstime.
Often there are photo request on the forum. Maybe there are people requesting something you have in your portfolio.
And what about the assignment? I've had some photos accepted in some assignments earlier. I found out that those photos get allot more attention and more views then my regular photos. And for me, those photos even got more sales.
For example this one on the right.
So I think, if you are active on Dreamstime with the Blogs, forum, assignment, etc, it's definitely possible to get more sales. And every extra sale is welcome.