Your artwork is not rubbish! I do just think it's the photos that could do with a little work, I'm not sure it's the surround that's the problem though but the way they're cropped. Such as you have a lot of images that are tall, which on the thumbnails makes them look a little cropped.
I just did a little messing around in photo editing software with one of your photos to show how the thumbnails could be.
1 is how it currently shows.
2 is if you added a little more surrounding so the image you upload is square. Because this is the same shape that Folksy shows their thumbnails nothing gets cropped off and the border shows all the way around the image, instead of just to the sides. I also turned up the brightness/contrast a little bit in the photo software too, as I always do on my photos or they come out a bit dull (but not so much it misrepresents the item).
3 again has the brightness up a smidge, and is if you cropped the image right to the very edge of your picture with absolutely no background showing (it would show the full tall image in the listing but crops a little off the top and bottom on the thumbnail). It still shows enough of the image to look good and not like there's something obviously missing, and I think I'd be more likely to click on that.
Your paintings are great! Just spend a little time with your camera and find some photo editing software to have a play around on. Remember what Folksy shows on the front page is square, so you need things to show as well as possible when they're cropped down to that shape.