A system for updating a sign-on logo image file in a computer includes: a storage medium (7) for storing image files; application software (1) for loading a selected image file, converting the selected image file into a designated format image file, and compressing the designated format image file into a corresponding sign-on logo image file; a dynamic link library (DLL) (2) for receiving the sign-on logo image file from the application software; a basic input/output system (BIOS) (4) for receiving the sign-on logo image file from the DLL through a driver model (3) which connects the DLL with the BIOS, determining whether the sign-on logo image file is valid according a size of the sign-on logo image file, and updating the sign-on logo image file in the computer if the sign-on logo image file is valid; and a read only memory (5) for storing the sign-on logo image file. A related method is also disclosed.