Custom Backgrounds are pictures that are either .png, .jpg of .gif format (max resolution is 240x180) that can be used to set a level's background using an URL. This allows for level creators to put their own background/theme into their level, allowing for much more creativity. This feature was added in SMF2's 2.1 update, so only hacks that are at least using 2.1 as base have custom backgrounds supported.
Custom Background databases Edit
Official Custom Background sources Edit
Level Palace Custom Backgrounds Edit
As an official part of Level Palace, users can create, share and upload all kinds of custom backgrounds to its Custom Backgrounds page, and is moderated by staff. It is also possible to search for tags to easily find the background you're looking for. https://www.levelpalace.com/cbgs
Other official Custom Background sources Edit
Some hacks come with a list of custom backgrounds to be used when you go to their respective page (you can find it by clicking Play here next to a game on Level Palace).
External/Unofficial Custom Background sources Edit
There are many users who host CBG's of good quality on a specific website.
How to import a Custom Background into your level Edit