There are now a few ways to do this. Firstly, you can select an image to be used as the background of any HTML item now Freeway Pro only). This is great if you have a textured image that can be repeated horizontally and vertically.
Select your HTML item
Open the Inspector and click the second pane (Appearance Settings)
Select an image in the Image pulldown
You can specify how you want the image to behave using the Horizontal and Vertical options that appear once you have selected a graphic.