Cloudup is one interesting service from Automattic which offers a simple way to capture screenshot with a shortlink. You can also upload images, videos, music, pdfs, etc. and quickly convert it to a shortened url. One nice feature it has is the “oembed” which automatically show a preview of its contents when you paste it with software that supports it like the WordPress editor (see sample below).
Best this is its free for the 1,000 items that you upload so signup now: https://cloudup.com/
Hopefully in the feature they can add an ability to put text comments on top of the uploaded images similar to the screencast software.
(Image credits to site/product owners)