To change your fiori launchpad logo you have to create your custom theme using the Theme Designer (/UI5/THEME_DESIGNER), there you can upload your own logo and background.
But if you face problems in the image upload then you can replace the logo by uploading your custom image with the same name and replace the original logo in the mime.
to do that:
Go to (SE80 -> MIME Repository) under the path /sap/public/bc/ui5/LIBRARIES/VER/lib_version/com.sap.ui5.core/META-INF/resources/sap/ui/core/mimes/logo/
then replace the logo image with your custom one.