It very much depends, was the unhelpful reply.
If you want a common graphic on every page, making your mind up on its size is your first port of call.
There's no real hard and fast rule about how big a banner image should be. I normally make mine around 350 to 450 pixels wide by between 90 and 130 pixels high, as a rough guideline.
A lot depends on what you want to put on the banner and which graphics program you have. In general though, anything of around the above dimensions saved either as a JPG or preferably a GIF file will load quickly and still look good.
Right click on your current image and select properties. Check out its dimensions and see how big it is.
If you insert an image in FrontPage you can call up the image properties in relation to the page it is on and alter the displayed size. Be warned though, it's a very poor habit to get into.
As an example, let's say you have an 800 x 400 pixel JPG image. This would be a reasonably large file and would take a little while to download and display online. A lot of newcomers to web design make the mistake of resizing their images in the web editor - that is, you use FrontPage, Dreamweaver or whatever to display the image at, say, 400 x 200.
It means that the large original source image still has to download online and you waste that download time by showing it as a smaller picture.
Doing things the other way, by setting an image size to be larger than the original on the web page often results in poor image quality since you lose some detail whenever you enlarge a picture.
I'd suggest you try and recreate your graphic at a suitable size for a page banner or at least check out the original file and see what its dimensions are. You can right click on the image in FrontPage and select Properties and see if a specified height and width are selected or whether it is being displayed in its native size.