@ soopa
You might want to try and put the images in a folder named "triiforce_mrc" on the root of your USB drive itself.
If there is a readme with the version you´re using, read it.
Depending on the versions you use, some want 192x112.png images, and others want 140x112.png images. I´ve even got "192x112", and "140x112" folders inside the "triiforce_mrc" folder. I think I´ve even seen some with 170x112 or something along those lines,but.... I´ve tried so many I don´t even know which ones needs what.
Anyway, If you don´t have a readme included, try mixing and matching with only one or two images just to see. I´m sure you´ll find what´s needed soon enough.
Have fun tinkering around, and good luck, if no one offers up a 100% answer before that.
(edit) no idea about the black screen, sorry. I know some games just won´t work with triiforce, check the compatability list, and some need a save placed on the emulated nand. A few just seem to take a really long time to load up.