I noticed the other day that post sizes on the Tumblr Official Theme are sized at 500px, which ends up compressing any photos posted on the blog that are originally posted at the dash post size of 540px. Going through the coding for the theme, I can’t find where to fix or change it.
I’ve tried looking through both the html and css coding in the theme but for some reason still can’t figure out where to make the changes.
The default Tumblr theme uses an external stylesheet for it's CSS, so you won't find much CSS inside the theme editor to edit.
The posts are contained in an element named .main, and the stylesheet has it set to .main {max-width: 502px}.
To change this, simply add .main {max-width: 540px} to your css (anywhere above </style>). You can change the 540 to whatever value you want.