Unless you've uploaded a high-resolution theme to your blog, your Tumblr layout most likely limits the size of images to a maximum width of 500 pixels. If you frequently post high quality images, however, you can modify the page code and swap out the "{PhotoURL-500}" tag with the "{PhotoURL-HighRes}" tag. The former renders pictures with a maximum width of 500 pixels, whereas the latter renders the photo with a maximum width of 1280 pixels.

Step 1

Select the "Settings" icon from the top right after signing in to your account, and then choose your blog from the left pane.

Step 2

Click "Edit HTML" in the Customize pane and then press "Ctrl-F" to access the search function.

Step 3

Type "{block:Photo}" into the search box and then press "Enter" until you locate code that resembles the following:

{block:Photo} {LinkOpenTag}{PhotoAlt}{LinkCloseTag}

Step 4

Replace "{PhotoURL-500}" with "{PhotoURL-HighRes}" (without quotation marks).

Step 5

Click "Update Preview" and then click "Save" to enable high-resolution photos in your theme.

Step 6

Click the back arrow and then click "Exit" to return to the previous page.