gnome-web-photo & php

It looks like the app won't actually print to the screen the way you've invoked it, so maybe you can get away by setting the DISPLAY environment variable first:

<?php
shell_exec('DISPLAY=0:0 gnome-web-photo -t 0 --mode=thumbnail -w 512 -s 256 http://www.google.com /tmp/google.jpg');
?>

I did a stack trace on gnome-web-photo.

open("/dev/tty", O_RDWR|O_NONBLOCK|O_LARGEFILE) = 3
close(3) = 0

I'm not sure why, but it really wants to open the terminal.

If your goal is just to generate a thumbnail, have you considered using imagemagick's convert?

The API is here:
http://php.net/manual/en/book.imagick.php
Simply, Imagick::thumbnailImage should do the trick.

If you wanted to still use imagemagick on the command line, then something akin to:
convert -thumbnail 100x100x0x0 original.jpg new.jpg

or convert -resize works, too.