First of all, you must choose an image which you like the most, and click on the heart icon of your choosen backdrop treeline, if you want to give me some feedback.
You can see numbers under the title, this is the dimensions of the picture (the first is the width the second is the height). The more the first number is, the longer the backdrop will be.
Check the checkbox.
Image transparency works in EEVEE since the 2.81 release but the shadows are limited.
If you want to add some curved shape to your teelines follow these instructions.
You can add more loopcuts, if you scroll with your mouse wheel.
Chose the edge select option.
Press 7 on your num pad for top view, then hit 5 to ortographic view. After that, press G to move theselected edge. Scroll with the mouse wheel to change the size of your proportional circle.
Change the shading to smooth to get the best result.
This is the result with HDR lighting.