You go into your folder - mark the drawing - choose "share" - then choose "copy link" (if this option didn't appears, you have to "create link").
When you place the copied link in your sb post, it will looks like this example:
https://www.dropbox.com/s/h9mhllul83e9f ... 2.png?dl=0
You have to alter one thing! The zero at last to an one like this:
https://www.dropbox.com/s/h9mhllul83e9f ... 2.png?dl=1
The difference is shown here. With the end of the adress with "=0" it looks like this:
With the change to "=1" it looks like this: