How I sell my drawings and paintings in microstock (my personal experience with Dreamstime agency)

Yes, they do sell well. I was surprised too. I have no idea who buys them and why they're so popular, but facts are facts. There's a small collection of Children artwork , just for example. Cute and simple ones.
If you have some kid's drawings in your family archive, that's what to start with.

When you sell an image, DT allows you to see the keywords buyer used to find it. "Sketch", "pencil sketch", "drawing", "pencil drawing" are very common words for my sales.
If you have a pile of sketches collecting dust on your table, give them a second life: scan and upload.
I love sketches. First, I upload them to DT as they are, then I color them using Photoshop (many other programs will do too) or some real things (crayons, paints, markers, etc) and upload a colored variantion (see my Colored sketches collection ). I also can make collage pictures combining sketches with photos, sometimes it gives a nice result worth of buyer's money.
What to say... improvise! Sketches are great for that!

They sell well too. I don't have many watercolor paintings in my portfolio, but I know people who have portfolios made almost of watercolor pictures only, and their sales are just great.
And a funny thing about watercolor pictures that sell are so called "watercolor backgrounds": scanned pieces of paper stained with watercolor spots, lines, blurs, etc. They sell too.

Ahh... so-called digital art... It's a pity that some big paintings made in Photoshop take so much time and effort, and then sell so bad or don't sell at all :( But I can't make my judgment on them because I have several exclusions that sell great. In fact, my bestseller "Jungle road" is a digital art image.
So, I suppose, whether your complicated digital paintings will sell or not is a matter of luck... or destiny no one can predict ;)
PS: simple digital drawings (especially contour) sell much better. Just a hint: if you upload some contour drawings, add these keywords to increase sales: "coloring book", "coloring page". Most of my contours sold were found with them.

I don't like vector very much, but I have some Vector images in my portfolio. Can't say they sell good, though. DT appears to be oversaturated with vector images, maybe that's the reason (or my vectors are simply bad :D ).
Anyway, if you are vector artist, have in mind that you have some benefits. First, you can upload your vector image twice: as a jpeg (necessary) and as an additional format (which can be eps, ai, cdr). Not only each upload brings you 20 cents (if you're exclusive to DT) but additional format is more expensive for the buyer (and will bring you twice more money than biggest jpeg) and it's really making a difference.

I love them. But it's a game of chance, just like with complicated digital art. Some collage pictures sell surprisingly well, some don't even have views. But who knows when their time will come.
Make collages of sketches, photos, scanned textures... It's a very creative work, giving unexpected results sometimes.
When you have your photo rejected by agency's editor, give it a second life as a collage. This one - "Cat and fairy bird at night" - was made of four photos which were so amateurish and casual that I didn't even dare to submit them. And what a magical collage they became!.. not only magical, but selling good too ;)

Definitely, that goes into "must have". Frames are among my (and not only my) bestsellers. Draw them by hand and scan then, or draw them in vector, or make them from photos and collages: they will find their buyers. See my "Frames" collection for example.

You know, it's a funny way to use scanner. I don't know why some people still photograph flat objects when they can be scanned :) Floppy discs, your granny's old buttons, pins, rough paper and cloth textures... (and even bugs!) - continue if you wish. Textures and objects isolated from scanned images sell great (see some of mine there).

I'm a poor photographer, and photos are scarce in my portfolio. But just looking through the portfolios of others and comparing photographers' and artists' sales I can say: drawings usually sell better than photos. Maybe there are too much photographers around and not too much artists, who knows...
But photographers have a benefit too. Illustrations almost never win DT assignments, photos take all the prizes ;)