A: Yes you can!   If you have ResHacker you can replace it thusly:  Change the extension of mypic.bmp from .bmp because ResHacker doesn't see it right otherwise.  Then in the Petz 4 Rez.dll (or Petz 3 or Petz 5 of course) find the Wizard resource under BMP in ResHacker, right-click and choose Replace Resource, browse to find mypic.bmx (or whatever you renamed it to) and click on it, then fill in BMP under Resource Type, Wizard under Resource Name, don't bother with Language unless you want to -- that would be 1033 if you do -- and then hit Replace.  Don't forget to Save after that.

Anyway, that's what the big deal is as regards ResHacker.   Please note that with it you can only edit Windows 32-bit resource files (.dog and .cat, .clo, .toy and .env files are all resource .dlls in disguise) and not 16-bit (Oddballz, Catz 1, Dogz 1).  Also, you cannot edit .pet or .baby files because these are not resource files; they are save-game files.