Photos and photosets in posts shown in the Tumblr home page Dashboard stream display as thumbnails by default unless specially modified by users when originally posted. Although you can click any thumbnail to see the photo at full size, doing so repeatedly takes time. If you prefer to see all photos displayed automatically at their largest available sizes, change how they appear via your account Dashboard settings screen.

Modify the Settings

Click the gear on your Dashboard to see the settings options for your account. Select “Dashboard,” select the “Show Full Size Photos” option and then click the “Save” button.

Warnings

Photos and GIFs within notes located beneath a reblogged post, or Ask question post, won't display at full size when you change the Dashboard settings. You must still manually click these images to enlarge them.

Disclaimer

Information in this article applies to Tumblr as of December 2013. It may vary slightly or significantly with other versions or products.