Click on the Stylish icon in your taskbar, and go to Manage Styles. Look for OCR4 Dark Style (1280x1024 or 1024x768, whichever one you downloaded), select it, then click Edit. Right-click, select all text, then copy it into Notepad (or whatever Ubuntu's equivalent quick text program is) and use that program's Find Text feature to find the http://ramaserv.thasauce.net/images/OCRCSS/OCR4-Contrast/OCR-Contrast-Logo-03.png. Change it to http://ramaserv.thasauce.net/images/OCRCSS/OCR4-Contrast/OCR-Contrast-Logo-04.png. Then copy everything back into the Stylish window, hit Save, and the site should be instantly updated with the new header.