This tool is made to work with BIN files made with image-to-BIN converter.
These BIN file use 3 bytes for each pixel: Blue, Green, Red. No alpha transparency.
Read the 3DS homebrew tutorial to learn how to use these BIN files in your homebrews.
Set you image's width here:
Set you image's height here:
Press this button:
Right-click on the image and download it: