Q: I wanna link to another submission via thumbnail in my text/description, how do I do that?

In your story text or image description, put the following text:

:link9999:

The number 9999 is the page ID of the submission you want to link to. You can see the page ID in the URL of the page. If the submission you link to is an image, it will be displayed as thumbnail, otherwise a music icon or user avatar of the author will be shown.