What is the difference between ‘image post’ & ‘text post’ in feed content?

You can create two kinds of posts using RSS Feed content:

1

Image Post

The image post is the one where you select/upload an image to go along with your post.
For example, refer to the screenshot below. 

2

Text Post

A text post is the one wherein you include a link but don't select/upload an image.

Clicking on the image in this post will take you to the link.