From The Apple Wiki

GlyphCharging is located at /Firmware/all_flash/all_flash.(MODEL-NAME).production/(GLYPHCHARGING-IMG3-FILE-NAME).img3 in the IPSW and flashed to NOR (or a functionally equivalent partition). It is displayed when the battery is too low for the device to boot up (under 5%)and external power is connected and charging the battery.

It probably is a PNG picture packed into an IMG3/IMG4 archive. No (user-friendly) method of replacing this icon is known.