> code parses the question content which is in HTML and looks for <img> tags and extracts the image
This is not so technically difficult.
> If multiple image exists the thumbnail comes from first image the parser finds.
Because first appearing image may not be image related to the question, this may have some problems. For example, when we add external link in post of Facebook, images of page are extracted. And, we can choose those images. if he expect more accurate processing, these feature will be necessary.
Because there become many requests of the image, question list page performance may worsen. I forgot which version it happened from, but Gideon turned by default off the avatar display to questions list of Q2A. I think that he did not to worsen the performance of the question list. But these problems may be solved by taking image from server with Ajax. I do not have confidence. . .