This issue has been found in CA PPM 14.3.
This issue has been reported as CLRT-80063 / DE32390 to Engineering.
This issue has been fixed in CA PPM 15.3.
If the icon has already changed, it might be necessary to change it back to the Timesheet image after the upgrade to 15.3 (or higher) by doing the following:
Go to Administration > Studio > Portlets > Find and click on [My Timesheets] link (cop.invMyTimesheets) > List Column Section > Fields > Timesheet > [Properties] icon > and select the "Timesheet" image.