If you put the image as a background in html, then you need to add that line of code with the other fo.addparam:
If you use the backgroundImagePath option in gallery.xml, then your image needs to be as large as your window and you need to place the logo at the right place in that image.
Get in photoshop and create an image that will be 1024 pixels wide with the right background color. Then add your 250 pixels logo on that image at the right position, and save the image. And ''voila'' ... Your background image will be as large as your window and your logo will be at the right place.
Remember : if you change the size of the clip (not using 100%), then your background image will not fit anymore.
It all depends on what option(s) you are using and the way you setup your page. I prefer to use fixed layout because this is giving me more control on the way the clip will look. Doing so i have to let go the automatic resized. However, if you choose to run without fixed layout, then it might look good on your full screen with your current resolution, but might also look very strange in another browser or with a different resolution.
You have to test your clip under different browser and different resolution to make sure everything is always ok all the time.