Your right but you can modify the image to have a seamless border, this means you can not see the difference when the image repeat itself
Better you could do a seamless tiles and use the repeat background (a tiles 64 by 64 by example, you could find tutorials using photoshop or gimp to create the tiles).
You can modify the .tpl file and create a new centered div (wrapper) which contains your image but again if the screen resolution is higher than the div you will have borders.
You can also add the image in the background div itself.
In any case if you have always an image in the middle (the girls at the party or other thing from prestashop) you can remove this part from your background so this background image will be lighter.
The link given by cocothecat is useful.I bookmark it.The CCS3 key "cover" is new for me
Concerning Cpanel I don't think it is designed to create or modify a website, personally I'm using notepad++ , firefox and firebug, gimp and/or photoshop.
Cpanel is strong to modify rules on the htaccess file without touching it directly and to see statistics and logfiles.
Again if you want to see how a website is done using firefox and firebug is really useful.
PS : cocothecat something seems wrong on your website, the html code appear clearly visible close to the panda and rabbit image on firefox. PM me if you want a screenshot.
PS : I'm also learning the web design and php, just created two websites but none with prestashop at the moment (only local test).