[VM] Don't display images unless alt text exists

[Yeechang Lee]

I have set up VM 8.1.2 to do a great job in displaying HTML messages
with emacs-w3m. I use a text terminal, which means emacs-w3m turns
images into text "buttons"; if there is no alt text, the text is
constructed from the filename of the image. Is it possible to only see
images (whether ordinary images or links) if there is alt text? In
other words, if there is no alt text, I'd rather see nothing than the
gibberish of images' file names.