What operating system on your computer you are using?
Are you able to install imagemagick?
http://www.imagemagick.org/script/binary-releases.php
see those threads
https://www.mobileread.com/forums/sho...ht=imagemagick
You can also use irfanview as described in the first post in this thread
https://www.mobileread.com/forums/sho...ht=imagemagick
I am using escher.png as an example, but you can use any format, not only png as a base for conversion with convert program from imagemagick package, even bmp.