You have made changes to a PNG file, but the modified date (date.mod) field of the PNG file is not updating within the Xinet browser. You see the "updated" date field changing within your file browser (such as Finder) but not within the Xinet browser.
This behavior with the metadata not updating is isolated to older versions of Xinet due to the version of the PNG library (libpng) that they have packaged. The Development Team addressed this issue by updating the PNG library version to 1.5 within the 19.2r2 release of Xinet. In later releases, PNG files can now have their Metadata updated, as with other file types.
To begin utilizing this feature, update your Xinet Server to 19.2r2 or greater. See Downloading Xinet Installers for guidance on acquiring an up-to-date version.