Tumblr actively encourages its users to reblog and share posts from others, but there's a mechanism built into the platform to acknowledge the original source when this happens. Most Tumblr themes include the necessary references to show the source of a post, but if the information isn't displayed automatically then you can add the code manually.

Editing Theme HTML

Click "Customize" from the Tumblr dashboard to find your blog's customization screen. You can then click "Edit HTML" to make changes to the code of the current theme. Making any modifications disables the auto-update feature in order to avoid overwriting your changes, so any future releases of the theme must be installed manually if you want to apply them. Use the "Update Preview" button to see how your alterations will look on your blog.

Source Variables

You can use the variables "{SourceURL}" and "{SourceTitle}" in the theme code to stand in for the URL and title of the post source. You can also use the "{block:ContentSource}" and "{/block:ContentSource}" tags to contain source information, should you want the metadata hidden when no source is specified. If you want to display the source's logo as well (typically the Tumblr blog's avatar) use the "{block:SourceLogo}" and "{/block:SourceLogo}" tags to contain the information; the "{BlackLogoURL}", "{LogoWidth}" and "{LogoHeight}" variables refer to the logo URL, its width and its height. Study the theme's existing code and the Tumblr custom HTML documentation (see link in Resources) if you need help placing the variables.

Post Source Information

Whenever you reblog a post on Tumblr, the source information is already filled out for you in the new post. Click the cog icon in the top right corner of the post creation window to double-check the information or to specify a source if you are creating a new post from scratch. If a source is not specified when you publish the post, then the theme you have installed won't be able to display the information on screen.

Reblogs and Sources

A post's source can be anywhere on the Web, from a Flickr page to a Wikipedia entry. Whenever you reblog a post on Tumblr, the platform also includes information about which blog you reblogged the post from and the blog that originally published it. A post can therefore have three bits of information tied to it: the original source of the content, the Tumblr blog that first posted it, and the Tumblr blog it was most recently reblogged from. The original Tumblr can be attributed using the "{ReblogRootName}", "{ReblogRootTitle}" and "{ReblogRootURL}" variables, while the Tumblr the post was reblogged from can be identified with "{ReblogParentName}", "{ReblogParentTitle}" and "{ReblogParentURL}". In both cases the variables return the blog username, title and URL respectively.