If you regularly post photos to your Tumbelog, the fact that Tumblr resizes photos to fit your blog format can sometimes get frustrating. You may want your followers to view certain images in their full glory, meaning in full resolution. While you cannot technically prevent Tumblr from resizing the image on your Dashboard and main Tumblelog page, you can enable hires viewing so that your readers can click to see the full version.
Navigate to Tumblr.com, and log in to your account.
Click the name of your blog at the top of your Dashboard, then click the Customize Appearance link in the right column.
Click the Advanced tab on your Customize page.
Check the box next to "Enable hires photos." This will allow your readers to click your photos to see a high-resolution version, but keep in mind that this will not apply to photos with a click-through link.
Click the Save + Close button in the upper-right corner to apply your changes.
For larger photos on your blog's main page, choose a theme with one wide central column. The width of your central column reflects the maximum photo width that will appear on your blog's main page.