Toolbar icon does not change when replaced

I changed the icon on the footnote command on the standard toolbar. The change HAS NOT flowed through to the toolbar, though the new icon IS displayed on the customize-toolbar-content menu. I’ve closed everything up and shut down LO 5.3.4.2 several times hoping that the change would be digested but no. Is there anything to be done? Yes, the new icon is .png and the proper size. Thanks in advance for your help.

Thank you so much. It works.

If you want to change an icon, then you first need to go to Tools>Optione>LibreOffice>View and set Toolbar Icon size: to Small. Then go to the customize dialog and assign a 16×16 icon. Then back to Toolbar Icon size: and set Large. Again to customize dialog and assign a 24×24 icon (or 26×26 in case of theme Galaxy). At last step you can go to Toolbar Icon size: and set it to Automatic (which is the default).

Toolbar icons cannot properly be assigned in mode Automatic.