The entire Designer category (and Default category) is missing from Widgets in 4.13 and thus, is impossible to add a preview background to widgets. Widgets created prior to 4.13 retain their preview background but it can not be changed.
How it should look: (Taken from unreal documentation)
How it looks:
Hello Juice-Tin,
I will put in a report to get the documentation updated to reflect this change. You can very easily get around this issue by adding an image with a screenshot of your game applied, setting the anchor points to be fullscreen, and zeroing out all the values (see screenshot). To be safe disable the image under the Behavior tab if you plan to have it appear in-game you shouldn’t have to do this if you only plan to use the image for reference.
EDIT
Upon further investigation I’ve decided to log a bug report rather than contact documentation. The report has been logged here: Unreal Engine Issues and Bug Tracker (UE-36441)
Thank you for submitting a bug report,
Just noticed that the designer category is still available in the details panel when in the Graph mode, so it can be accessed through there for now.
Good find! This doesn’t seem to mentioned in our documentation anywhere so this is either a bug or a lack of documentation. I’m going to add this info to the bug report since this change seems unintentional.
for leaving that info hopefully some other users will benefit from it.
THANK YOU man! you’ve made my day 
Oh wow, this is quite obscure. Thank you for posting an explanation on how to get to it.
