I deleted the "Minimise all windows" icon from the taskbar, then restored it this way - ymmv:
Note: I am using the Mint-LXDE session at login - not the Default.
Add the "Minimise all Windows" applet:
Right-click the Panel
Panel settings > Panel applets > Add
Click "Minimise all windows" > Add
Move it to desired location as usual:
Right-click the icon
Panel settings > Panel applets > click Minimise All Windows icon
click Up button until moved as desired
Close window.
Go to /home/yourusername folder
click View > Show Hidden
Navigate to:
/home/yourusername/.config/lxpanel/Mint-LXDE/panels
Open file "panel" -
make sure you have user permissions to edit the fileScroll down to section "Plugin ... type = wincmd"
I edited that section to look like this:

- PANEL.png (33.91 KiB) Viewed 1351 times
save file
close
logout and log in
EDIT: I chose this particular icon, but if you wanted the
default "Minimise All Windows" icon,
the edited "image line" in the file should read:
- Code: Select all
image=/usr/share/icons/Mint-X/places/22/user-desktop.svg
caribriz