When I upload a background picture, it doesn't fit the page properly........

The default background pictures provided in Drupal Cloud are 800 x 533 pixels. You'll probably want to use a picture that size or larger for a single background image.

To adjust the positioning of your image, you can adjust the CSS in the background image settings tab (admin -> Configuration -> User Interface -> Dynamic Background -> Settings, then click Advanced - CSS Behavior).

On one of my sites, I used the following CSS (on the .html selector) so the image would automatically stretch to fill the entire window (on modern browsers):

background-position:center fixed; background-repeat:no-repeat;
  -webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;