It's possible to upload a top bar graphic on all pages. The height of the graphic is variable while the width is fixed. However, the width can vary in different skins. Below you will find the four different top bar widths.
Top bar graphics
In D4 you can use a top bar/header image that will display across all your pages. The height is variable and the width is 900 pixels. It's important that you only use this area for a logo (save it as a transparent .gif or .png if possible) as using regular images will conflict with your background.
In most skins the top bar width is 900 pixels or 956 pixels and it is obvious to see if the width is 900 or 956. If the top bar width is 900 it has the same width as the content area. If the width is 956 it extends a little beyond that area. Try to upload a test image to see the widths in action.
In D3 skins there are four top widths: 900. 956, 976 and 972 pixels. You can find the 976 pix width top bars in D3 skin w4.1-w4.6 while the 972 pix top bars can be found in skin b4.1-b4.6. In the rest of the skins the top bars are 900 pix or 956 pix.