Jerboa’s not too bad, as the URL field is the same field the link to your uploaded image ends up overwriting, but I swear I see people making this mistake every fucking day. End up posting just an image when they wanted image + link.
Apologies, as I’m sure there’s proper channels for this, and open source projects tend to not have the best UIs. Just shouting into the void.
That is possible
To get a link post with a custom thumbnail, you add the link and put the image in the thumbnail field.
To get an image post with a link, you make an image post and put the link in the post body.