Create Custom Feed

I would like to create a custom feed for my blog that I will use exclusively for publishing the existing blog posts on a different non-blog site. Because this feed is for my internal use only, it does not need to follow any specification (rss, etc).

All of my blog posts begin with an image.

I want to include a separate field in the feed that contains a link to the thumbnail version of the image for each post.

Any help here would be appreciated!