I assume that you still have the .X3D file that you used to create the .SWF file...

If you do, open the .X3D file and open up Texture Options.
There should be a drop down there, change it to Background.
Once that is done click on the button that is labeled 'Load Texture'
Navigate to where you have your background photo, select it and press the 'Open' button.
Done.

You can also then move the photo around a bit using the X and Y sliders.

I hope that helps,
Eric