Definitely link back, at the minimum.
I ran into this when I decided to use Etsy products to tell a story in this post:
http://stitchingincircles.blogspot.com/2011/02/skunked.htmlI let the etsy sellers know I had used their image of their product and had linked back to their shop. Most of them were pleased; HOWEVER, the person who was selling a *photograph* was less so. In that case, the image is their product, so I can understand that. The photographer said she would grant permission, but that the etiquette is to ask before posting.
So, for people who it doesn't hurt to have their image copied and pasted, I'd say ask afterward, but if you are using an image from someone who makes their money producing images, be cautious and ask before posting.