You can check an example on my personal blog. This tutorial may be a little bit difficult if you don’t have any HTML experience.

01. Go to Customize > Edit HTML. Now you have to find this code: <img src=”{PhotoURL-500}” alt=”{PhotoAlt}”/> or something really similiar to this. This code is usually between {block:Photo} and {/block:Photo} tags. There’s an example:


02. Now, you have to put this code: 

<a href="{ReblogURL}" target="_blank">

BEFORE the <img src=”{PhotoURL-500}” alt=”{PhotoAlt}”/> code and put that one: </a> AFTER that. It will be like this:
 

03. Click Update Preview and if it’s everything okay with your theme, click Save. Go to your blog and check if the code is working correctly. Done :)