Log in to your Tumblr account, and then click the gear icon to open the Settings menu.
Select the blog you wish to customize, and then click the “Customize” button in the Theme options.
Click “Edit HTML” under your theme’s name in the theme customization screen.
Press “Ctrl-F” in the HTML field to bring up the search function.
Type “block:photo” (omit the quotes) into the search field to search your theme’s code for the instance of the “{block:Photo}” tag, which relates to how your theme handles image posts.
Enter the following line after the “{block:Photo}” tag:
{LinkOpenTag}
This instructs your Tumblr theme to use the high resolution version of your uploaded photo for image posts. If your theme already has a similar “{LinkOpenTag}” photo variable, replace it with this one.
Click “Save” to save your changes.
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