This page will explain:
I like to keep Anansi lean and mean so there's no preview support for gif and jpg files. And as long Anansi doesn't do true WYSIWYG, it would be a shame of the enormous increase in filesize that would be needed.
So, for the hackers: If you really want to test your document colors with your background image, do the following:
Open your favourite image processing application and make a bitmap copy of your background image.
That's it. You'll see your background images tiled in the Color Assistants.