You could use the google image grabber.
So if your tumblr blog is
blog123.tumblr.com
then you do
site:blog123.tumblr.com keyword
(or use without the keyword depending on how many there are)
and load that into the google image grabber and you are depending on google to have crawled the blog and indexed the images, but it most likely has. Also your using google so any ip ban rules would apply, so use proxies.