change first image in RSS without affecting post

I use Bloglovin to promote my blog. I would like to change the thumbnail of my posts there. Their response is:

In order to get your images on Bloglovin, your RSS-feed needs to contain images. The image displayed on Bloglovin will be the first image displayed in your RSS-feed, hence not necessarily not the first image in the post. The images needs to be .jpeg and no other file should be placed above the image in the RSS-feed.

So, how do I put an image on top of the RSS-feed without that image being visible in the post?

I thought to put that image in the post with class=invisible-img followed by .invisible-img {display:none;} in the stylesheet, but I’m not sure if RSS ignores that CSS.