Yes, this is possible. Here's the code you can use, using the image on your banner as an example:
background: url('http://trishpratt.files.wordpress.com/2013/08/tp-final2-reddle-wp-banner-grid-theme-venture-colors.png') left top no-repeat;
You will need to alter a few things here.
- Upload the image you want to use, then get its URL. Use that URL to replace the URL within the url(...) part in the background parameter.
- The padding-top value will need to be changed to something close to the height of the real image to use. So if your tag line image is 100 pixels high, replace the 300px to 100px (or something around that value, if you want to play around with the spacing).