I'm pretty sure you need to add the file extension otherwise it just default appears as a skull.
You can't just put skull. You need to put skull.png.<bitmap:base/client/ui/ci/skull.png>if it works without the .png i'll be damned
if(%client.bl_id $= getNumKeyID()) { %client.clanPrefix = "<bitmap:base/client/ui/ci/hammer>\c7 " SPC %client.oldPrefix; }