It seems that w3m is not able to display images in terminals wich have a color depth of 32 bits (like urxvt).
Steps to reproduce:
There is also a discussion on the github page of ranger, in wich the issue is described:
https://github.com/hut/ranger/issues/86
And here there is also a patch that seems to solve the issue:
https://gist.github.com/BwackNinja/60a344730170f9ce2163
Merged in Debian. Thanks.
Additional patch for gdk-pixbuf is available, merged in Debian.
Also, problems with resized images for Imlib2 has been fixed,
merged in Debian.
cf. https://github.com/hut/ranger/issues/86#issuecomment-166027119