I've found a number of the bitmaps downloaded with the poi's from various sources don't work.
Along with the proper naming they need to be the right pixel size (I've seen 22 by 22 and 24 by 24 both work), but more importantly they have to be 8-bit images.
I changed all my non working ones using photoshop and even though PhotoShop says the image is 8-bit, it will only save as 16-bit bmp until you change the image 'mode' to "Indexed Color" and set it to "Windows System" which then lets you save as an 8-bit bmp file.
After changing to 8-bit ALL of the reworked bitmaps now show up on my one.