If you get the error PHP Fatal error: Call to undefined function Matomo\\Plugins\\CoreAdminHome\\imagecreatefrompng() it means that you need to install the GD extension for PHP. The GD extension is required for Matomo (Piwik) to generate report graphs, generate sparklines graphs, as well as to upload a custom logo. To check whether the GD extension is installed on your server, go to Matomo > Administration > System Check.