That's a feature of the browser, not GWT. You need to make the URL
variable. You could probably just add something to the end of it like
'?variable=1' and increment the 1 every time you need a new image.
The servlet will ignore the extra parameter (unless you programme it
to pay attention to the extra parameter, of course), but the browser
doesn't know that, so it will re-request the image. You might want to
use the current time in milliseconds, or something, rather than
starting at one every time in case the browser caches your image.
Tired of pop-ups, security holes, and spyware?
Try Firefox: http://www.getfirefox.com