I don't recommend hotlinking (I tend to extensively organize my images. and it could break your image if you hotlinked from me !!)
I use filegarden, tideous but usable
I add the credits to the file itself
tumblr@username
I add the credit to the alt as well !!
example:
Most of the graphics I put in my resource are from tumblr!
Tell me if any of yours/someone elses graphics were used and weren't given credit
For removal or credit, contact me here