Unfortunately, Tumblr may not always preserve the original dimensions of photos you post to your blog. Tumblr uses custom variables to define the maximum width of pictures you upload, and the blogging platform will resize a posted image if it doesn't meet the required dimensions. You can modify the variables used in your theme to increase or decrease the size of photo posts.

Supported Dimensions

Tumblr restricts the width of standard-resolution photos to 100 pixels, 250 pixels, 400 pixels or 500 pixels, depending on which tag is used in a theme. The majority of themes you can download from The Theme Garden display images at a maximum width of 500 pixels. Pictures can be smaller than the specified width, but not larger. For example, if a theme limits photos to 400 pixels wide, your blog won't make any changes to pictures that are 399 pixels or smaller, but will automatically size down images wider than 400 pixels.

Modifying Posts

To change how your blog renders pictures, log in to Tumblr and then go to your account settings (link in Resources). Select your blog from the left and click "Customize" to begin editing your theme. Click "Edit HTML" and search for "PhotoURL" using Find ("Ctrl-F"). Press "Enter" to locate code that looks like the following:

{LinkOpenTag}<img src="{PhotoURL-X}" alt="{PhotoAlt}"/>{LinkCloseTag}

"X" represents a numerical value. Swap out "{PhotoURL-X}" with "{PhotoURL-500}," "{PhotoURL-400}," "{PhotoURL-250}" or "{PhotoURL-100}," depending on what size you want your photos to be. Click "Update Preview" and then click "Save" to make your changes live.

High-Resolution Photos

If you frequently post large photos to your blog and find the standard tags too limiting, you can swap out the code with the "{PhotoURL-HighRes}" tag, following the same process described above. The "{PhotoURL-HighRes}" tag renders photos more than twice the size of the "{PhotoURL-500}" tag; Tumblr limits the maximum width for high-resolution images to 1280 pixels. You can also search for high-resolution themes from the Customize pane. Click "Browse Themes" and then choose "Hi-Res" from the drop-down menu.

Portraits and Photosets

Tumblr also restricts the dimensions of portraits and photosets. If your Tumblr theme allows you to use a profile photo with your blog, but you want to make your photo larger or smaller, search for "PortraitURL" in your theme to locate the "{PortraitURL-X}" tag. Replace the value to the right of the dash one of the following numbers: 16, 24, 30, 40, 48, 64, 96 or 128. For photosets, search for "Photoset" to locate the "{Photoset-X}" tag and replace the value to the right of the dash with "700," "500," "400" or "250." Each number specifies the maximum width of the portrait or photoset.