One image that certainly isn't the oldest but might be one of the oldest important images that is still online at its original location is the Blue Ribbin Online Free Speech Campaign animated logo (https://www.eff.org/files/br.gif).

This image dates back to 1996 when the EFF launched a campaign against the Communication Decency Act (CDA). Many of the pages across the internet turned their background images black in protest of the CDA. What remains of the CDA is today known as "Section 230".