You can change the welcome message in Settings -> Languages -> English. If you want to change the design around that message, you need to edit the template itself, not the welcome.php.
Basically, the template controls the general look of the application, but you cannot edit the individual pages themselves, as those files are encrypted.
However, there is a sort of workaround. Since we use jQuery by default on the site, you could use it to detect which page you're on (by looking at the "pageid" GET variable) and manually change the contents and design of the site using jQuery functions. Unfortunately this is not something that we directly support so you're on your own.
Alternatively, we do offer professional services to do custom work on installations of PrintShop Mail Web, however this does come at a price and you would have to contact the reseller from whom you've purchased the software to set this up.