What you can do to create a post for an image is to create a Description post.
Markdown supports images, but the images needs to be hosted on a website somewhere. The markdown syntax for that is like this:
So for instance, using the Image hosting service PostImages, you can upload an image there, then use the "direct link" to the image in market to do this:
PostImages will even let you "upload" from an image hosted on another website, which is how I transferred the @Stacker_News Twitter header logo (used in the image above) over to PostImages.
The title is optional, so you could also just do:
This works the same whether it is for a Description Post or for a Comment Reply.
