A watchpoint (also known in Mednafen as a watch address) is a breakpoint you can set, where the debugger halts execution when a specific part of memory (or I/O) is accessed, instead of just an execution address. You can set Mednafen to stop as the VRAM is getting loaded from the cartridge.
Which brings us to the second point - if you just look at VRAM, you may not be able to get the desired result. The VRAM is loaded from the cartridge (on some games, this happens multiple times because VRAM is often in short supply). If you want Doraemon's head to appear differnetly, you need to alter the source data on the cartridge, not just the VRAM data.
But having found it in VRAM is a big step; from this, you can find the cartridge data by doing one of the following:
1) Set a watchpoint on the VRAM write to that location, examine the loop where it's being loaded, and determine the source location of the data.
- or -
2) Now that you have a dump of the data, see if you can search the source ROM for the same byte sequence.
I'm not sure about tools to do the editing though.