Could it be a .bmp format not currently understood by NetSurf?
I just created a test html file that included a BMP image, and made an
If ImageFS2 has its action for BMP files set to Inactive, then NetSurf
renders the BMP image on the page.
But if ImageFS2 has its action for BMP files set to Active (which is the
normal state for most files on my machine) then NetSurf doesn't render the
image on the page (unless it has already done so).