Problems with custom portraits

So I've made a few custom portraits:

They are all 210x330 pixel 24-bit bmp's as required
They all end in "L" on the filename as required

They look fine on the character description screen but on the overall party portrait list they show the generic "Biff the understudy" image i.e. the generic face with a questions mark in the middle, without the portrait I created

Does anyone know why??

Thanks