for i in *.tiff; do sips -s format png $i --out ${i%.tiff}.png; done
via http://osxdaily.com/2013/01/11/converting-image-file-formats-with-the-command-line-sips/
also http://stackoverflow.com/a/9018877/362030
Then:
pbpaste | xargs -I{} curl -i http://git.io -F "url={}"