There are three ways that I know of!
My username on Polyvore is orreed
On all these you press the insert image button
Excited to see your designs!
1. On the first page their is instructions
2. That way confuses me a little. So the way I do it is click on my username. On the top is a slider with my designs. I copy and paste the URL. The only thing is they turn out quite small but I need to do some exploring because when I first joined they had a basic picture (you can not use this on the clickable ones) bigger.
3. Take a picture of it on Photobucket. This works but the quality goes down of the picture.