Hi there, I'm glad you like Superhero!
You can upload any image you'd like to use like under Appearance > Header. The theme is designed to best accommodate an image 480 pixels wide and 150 pixels high.
Are you comfortable with CSS code? You can change the font of the site title by targetting this CSS element:
font-family: Helvetica, Verdana, sans-serif;
You can replace the list of fonts above with any standard web fonts you like.
An easy way to add custom CSS like this is to install the Jetpack plugin and activate the Custom CSS module. You could also use a standalone custom CSS plugin. This way, your CSS won't be overwritten when you update the theme.
If you'd like a wider choice of fonts beyond what folks are likely to have installed on their computers, you could look into a Google Fonts plugin, which lets you choose fonts from Google's free font collection.