TL;DR: X11 itself does not have any concept of “icons” or a “minimized” window, but the ICCCM & EWMH (the standard for how X11 window managers and applications

TL;DR: X11 itself does not have any concept of “icons” or a “minimized” window, but the ICCCM & EWMH (the standard for how X11 window managers and applications should interoperate) does. Additionally, most window managers don’t use window icons anymore, but instead moves minimized windows to some sort of task bar, and do not, therefore, show any “icon” image which the application might have configured using the old standards.