Again update, tested to run on submitter's machine.

In current implementation, img_display._get_font_dimensions() queries the width / height (in pixels) of the tty through the TIOCGWINSZ ioctl.

This works in xterm and urxvt, but returning ws_xpixel/ws_xpixel from the TIOCGWINSZ ioctl is not mandated (for example, in all VTE-based terminals, like gnome-terminal and lots of others).  See also the following quote from tty_ioctl(4):

>       The struct used by these ioctls is defined as
>
>           struct winsize {
>               unsigned short ws_row;
>               unsigned short ws_col;
>               unsigned short ws_xpixel;   /* unused */
>               unsigned short ws_ypixel;   /* unused */
>           };

w3m itself uses the `-test' option of `w3mimgdisplay' to determine the pixel sizes of ttys.  Therefore, if we follow the same approach, we can (hopefully) display images in any terminal that w3m can.

Attached is the patch.  Happy hacking :)