click the edit button and then click the image icon under the title on the right side. It will allow you to re-upload it without deleting the comments and likes.
There is a little image icon under the URL field - I think that's what you meant. I uploaded it that way, and the thumbnail showed up. I also added some text inside the brackets like @soupspoon suggested. So, one of the two suggestions worked, thanks guys!
From the markdown formatting that shows up instead of a thumbnail, I think that you clicked link instead of the little photo icon to directly upload a photo? Maybe it would show up if you put some text in between those brackets at the beginning? Not sure since I haven't tried to do a photo link yet
Eta: also meant to say, what a beautiful kitty! I can't wait for a million extremely specific cat subs to start popping up here, then you could also post in watercolor cats : )