As the title says, I can't find where to replace the title text with a PNG file as a logo in the header. Gizza clue!!
Assuming you already have a Logo content block on your site. If you don't have one you will need to add it.
Click Customize Design
Click Entire Site
Hover over Logo content block
Click the wrench icon
Click the Image tab
Fill in the details of your image
Click Done again
Select Publish Now on the Save Changes dialog