You can't have trans in a .jpg file, if you are to use paint.net for images, use the select wand to select the white area and hit Delete on your keyboard, but you might wanna make the note white after deleting the white background, or invert the colors then get rid of the black background, thus leaving the white note, but you need to save it as a .png then, and i think for it to work as a particle, it needs to be 64x64, 256x256, and some other numbers i forget... Look at the sizes of the other particles in base/data/particles to help you with sizes you can use
Edit: Here
