@samtheman26
It is optional of course. But to answer your question it is a grey area. The source URL is supposed to leave a link to where you found the content. Realistically, you can say you actually did find the content at the source URL you are using. Place the image there once, then remove it. You have a defense. It's all very loose. Social networks like Pinterest / Tumblr are made for sharing. Pinterest does it a little bit better because they have a partnership with Flickr which allows easy attribution, but nonetheless.
Generally, a lot of the content you're going to find on Tumblr already has many duplicate copies pointing to different locations, so I think you should be alright. But again, if you're not convinced then leave the source URL default and just change the description / #URL tag.