The file used to be logo.sys located in the root directory of C: drive, but now it's included inside io.sys and cannot be changed.
Fortunately, placing a logo.sys in the root directory takes precendence over the one inside io.sys, allowing you to have your own logo.
Simply create a bitmap file 320 x 240 with a maximum of 256 colours, and name it logo.sys. It should work.