The file you're looking for is likely C:Logo.sys. Just remove it to restore Windows 98's startup logo. I found this information on UseNet and am unable to confirm it at the moment, so give it a go.
Regarding your business process of reselling computers that have Microsoft Windows 98 installed on them: Be careful that you're not violating any license agreements by selling software you don't own. If the company you acquired the computers from did not also transfer the licenses or used those licenses as upgrades, you might be heading for trouble. I'm sure that you've already thought this through.
This was first published in May 2002