First off usually the distros name comes before Linux, if you even want to say that, so it would be Ubuntu Linux, Debian Linux, Gentoo Linux, etc. In most cases it is safe to just leave out Linux because that is understood. Second of all only Windows calls it a start button, for KDE it is called the Kmenu (most programs in KDE start, or have a "k" somewhere in the name) and Gnome I think they just call it the Application menu because that is where you launch your applications from
xd .
The best thing I can tell you, sorry I don't use Ubuntu, is your are looking for a picture somewhere in /usr. If you do manage to find it all you have to do is place a picture that you do want there with the same name. Beware of the size though if the new image is bigger than the Ubuntu logo then it may not work.
On a side note the Ubuntu logo looks better than the default Gnome one which is a giant foot ->
http://www.gnome.org/. Please note that by changing this you are taking away some of their branding that makes it unique, but at the same time this is Linux and in Linux you have the freedom to do and change what ever you want so enjoy and hopefully a Ubuntu user here in the Guild can narrow down your search more
wink