That means customizing the cpanel skin. If you make changes to the default skins, the changes will disappear with the next cpanel upgrade, so to prevent that you would have to copy a cpanel skin and give it your own name then customize it. That means with future upgrades, you may need to edit the custom one to keep it up to date.
If you just want to replace the image with one of your own, to work until a cpanel upgrade that changes the image used, the image you want to edit or replace should be
/usr/local/cpanel/base/frontend/[i:db29c17b6d]themename[/i:db29c17b6d]/images/cptop1.jpg
If you just want to replace the image with one of your own, to work until a cpanel upgrade that changes the image used, the image you want to edit or replace should be
/usr/local/cpanel/base/frontend/[i:db29c17b6d]themename[/i:db29c17b6d]/images/cptop1.jpg
I try to help when I can to make up for all the questions I've had answered myself. 