and put it inside the UIAtlases folder of your modlet. Then, you'll need to label your icons correctly in this folder. You can't just put the name of the icon. it has to be:
ui_game_symbol_YOURICONNAMEHERE
Some xmls like items.xml just use the icon name, but you still need the ui_game_symbol infront or it wont read correctly. Buffs.xml use the entire icon name, including the ui_game_symbol stuff.