Looks like arch suffix is needed

$ imgkit --install-wkhtmltoimage
/var/lib/gems/1.9.1/gems/imgkit-1.4.1/bin/imgkit:12: Use RbConfig instead of obsolete and deprecated Config.
Downloading wkhtmltoimage-0.11.0_rc1-static-amd64.tar.bz2 from http://wkhtmltopdf.googlecode.com/files/wkhtmltoimage-0.11.0_rc1-static-amd64.tar.bz2
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 10.6M  100 10.6M    0     0  23.8M      0 --:--:-- --:--:-- --:--:-- 23.8M
Installing wkhtmltoimage-0.11.0_rc1-static-amd64.tar.bz2 to /usr/local/bin/wkhtmltoimage
mv: cannot stat ‘wkhtmltoimage’: No such file or directory
chmod: cannot access ‘/usr/local/bin/wkhtmltoimage’: No such file or directory

...file wkhtmltoimage-amd64 exists (with the suffix).