In order to add your logo, first go to the Account page in your admin, by clicking the "Account" link in the top-right corner. Once you're there, click the "show uploaded files" link.

Once there, click "Choose File", and select the image file that you want to use for your logo.



Next, grab the URL of the image file that you uploaded. This can be found under  "Grab the url". 



Go to Apps > PixelPrinter. Select one of your orders. Click the "Edit" button next to Templates, and hit the "edit" button next to Invoice. This will open up PixelPrinter's template editor. 

Locate this line:

<h1>{{ shop_name }}</h1>

and replace it with:

<img src="IMAGE_URL_HERE" alt="{{ shop_name}}" />

IMPORTANT: Replace "IMAGE_URL_HERE" with the URL of the image that you copied in the Uploaded Files section.

You should now see your logo in your Invoice template! \(^0^)/ The same technique can be used for the other templates in PixelPrinter.