Hardcore mode backport from 1.0 to b1.7.3
Custom icons are only used for hardcore (non hardcore uses vanilla texture) and should be located at /assets/hardcore/textures/gui/icons.png so you should add 2 textures at those two locations:
/gui/icons.png - for non hardcore aka vanilla texture
/assets/hardcore/textures/gui/icons.png - hardcore only
Their layout is exactly same.