If you click "add meme" and select a template from the favourites list you can make your meme, generate and the "post comment"
If you are using a different template you need to;
1: search all templates for "single black line"
2: save single black line to "my templates"
3: when replying to a meme as a comment first select "add meme"
4: then pick "my templates" & "single black line"
5: now "add image under current"
6: upload from device or search meme templates
Now you effectively have 2 images working; a single black line that nobody can see plus your chosen image.
Hope that helps.