How to Stop Tumblr From Resizing Images
1. Log in to your Tumblr account and navigate to the Tumblr blog you wish to modify in your main menu.
2. Click on the "Customize Theme" button. The theme editing window will appear.
3. Click the "Edit HTML" button underneath your theme preview in the left pane.
4. Locate the "{block:Photo}-{/block:Photo}" tags within the HTML. The section of code you want to concern yourself with will look like the following example: {block:Photo} {LinkOpenTag}{LinkCloseTag}
{block:Caption}{Caption}{/block:Caption}
5. Modify the code so that the "PhotoURL-500" section reads "PhotoURL-HiRes," as in the following example: {block:Photo} {LinkOpenTag}{LinkCloseTag}
{block:Caption}{Caption}{/block:Caption}
6. Save your changes by clicking the "Update Preview."