The header image in the WP default Twenty Ten theme is 940 x 198 pixels. I wouldn't use anything wider than that.
I don't know what the IE8 problem is. It might be something in your code.
One thing could you could try is switch to the default Twenty Ten theme and then view your site using IE8 (and other browsers, as well).
Most Web designers test their site using several different browsers in order to see if there are any problems.
I've been using Photoshop for at least 15 years. That's what I use to create backgrounds and other graphic elements.
If you find a public domain (free) background or pattern that you like, try to save the image and use Paint (if you don't have any graphics problems installed) to adjust the image, make it more transparent, etc.