Trying to personalise the screen a bit. I'm not able to find a way of:
stretching the wallpaper to cover the entire screen
making the wallpaper adjust itself to display either in landscape or portrait

It's a bit rubbish really. The kinda bluey default wallpaper covers the entire screen, why won't mine? I've tried to upload the pictures in alll sorts of sizes/dimensions to no avail.

Because the blue original, is on Theme, and You try with a wallpaper. Only if You edit one theme, or do an new theme, with that wallpaper part of that theme.

Try http://www.ownskin.com (if you don't want to get into making your own themes using Carbide.ui S60 Theme Edition on Forum Nokia: http://forum.nokia.com).