umm lemme check to see what i have and I'll be right back
okay...my particular header is 860 px wide, 340 high at 72 dpi. I think you can go a smidge bigger if you wanted. Now to get to where you change it, log in to typepad, then
then there are tabs across the top, click on the one that says web blogs, then you'll get more tabs
manage/compose/design/configure
Click on the Design tab, then "Change Theme"
about halfway down that page you will see PAGE BANNER , with "edit this element" under it. Click that link and it takes you to a page builder page with places to choose colors, etc and then it will say "Optional" and you can upload an image from your computer there.
Let me know if this was clear as mud, lol