WARNING : I'm just going to say this now. DO NOT REPLACE GRAPHICS IN YOUR "GRAPHICS" FOLDER! THAT'S NOT HOW CUSTOM GRAPHICS WORK. (Unless you plan on permanently changing graphics.)
A lot of levels, almost all of them, have some sort of custom graphic used in it. Custom graphics are one of the many great features in SMBX, allowing for near-unlimited creativity. First things first is I recommend you download some custom graphics in the Graphics forum. If you want a large assortment, download a graphics pack!
For this example, I'll be using the Super Mario 3D World Graphics in 2D (found here :
http://www.supermariobrosx.org/forums/v ... =31&t=1959 ) .
First thing you'll want to do is go into your Worlds folder and make a new folder. Call if whatever you wish. I'll be calling mine MyFirstLevel.
Now that you have a folder, go back to SMBX and start a new level. Immediately save it to your new folder. Be sure to name it like you would name a level in an official Mario game. Now that you have that done, go back to your folders, and inside your level folder, make a new folder and name it exactly the same as your level. If you don't do this, the graphics won't be read.Now go find the graphics you want to use. Copy them, and paste them into your newly created folder.
Your level folder should look like this.
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Now head back to SMBX and re-save your file. The custom graphics will now load into the level and you'll be able to use them!
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Tada! Now you have custom graphics!