If you open the properties of a page, the configuration dialog of the page type will be shown. If the page type is "Picture" then you can choose a new picture for your page in the properties dialog and you can specify whether or not anti-aliasing shall be used when scaling the picture. If the page type is "Blank", then you can change the color of your page. If the page type is "Gradient", then you can configure the gradient parameters here. And if the page type is "Pattern", you can choose an other image file to use as the pattern.
If the page type is "Picture", you will also be able to choose a transparency setting for the image. This can either be an RGB color, pen-based transparency in case the picture uses a palette, or alpha channel transparency for image formats that support alpha channels like PNG. If you specify a transparency option, you will also have to configure a backfill setting for this picture. The transparent image will then be composited onto the specified backfill setting. See the documentation of the project properties dialog for more information on backfill settings.