Well, changing sprites and background tiles is something I'm familiar with at this point. More advanced coding is not.
A neutral dog sprite appears at 1C1C.
A happy dog sprite would look like:
00BA030000BB030800BC031008CA030008CB030808CC031010DA030010DB030810DC031018B7030018B8030818B90310
He could also jump out of his chair for the second frame, using the same tiles:
F6 BA 03 00 F6 BB 03 08 F6 BC 03 10 FE CA 03 00 FE CB 03 08 FE CC 03 10 06 DA 03 00 06 DB 03 08 06 DC 03 10 0E B7 03 00 0E B8 03 08 0E B9 03 10
Nurse sprite appears at 1C9F. With a palette change (and needing one tile redrawn) she'd be:
009501000096010808A401F808A5010008A6010810B401F810B5010010B6010818FF01F818C5010018C6010820D5010020D6010828E5010028E6010830F5010030F60108
Moved up and to the right a bit during a win animation, she'd be:
F5 95 01 07 F5 96 01 0F FD A4 01 FF FD A5 01 07 FD A6 01 0F 05 B4 01 FF 05 B5 01 07 05 B6 01 0F 0D FF 01 FF 0D C5 01 07 0D C6 01 0F 15 D5 01 07 15 D6 01 0F 1D E5 01 07 1D E6 01 0F 25 F5 01 07 25 F6 01 0F
(I might want to change her leftmost sprites slightly for this also.)
Garfield's neutral sprite turns up at 1B9C, but needs an extra first tile as follows:
00 47 00 00 00 48 00 08 00 45 00 10 08 49 00 00 08 54 00 08 08 55 00 10 10 59 00 00 10 64 00 08 10 65 00 10 18 69 00 00 18 74 00 08 18 75 00 10 20 83 00 00 20 84 00 08 20 85 00 10
A two frame Garfield loss animation would look like:
00 3A 00 00 00 3B 00 08 00 45 00 10 08 4A 00 00 08 4B 00 08 08 4C 00 10 10 5A 00 00 10 5B 00 08 10 5C 00 10 18 6A 00 00 18 6B 00 08 18 6C 00 10 20 83 00 00 20 84 00 08 20 85 00 10
00 3A 00 00 00 30 00 08 00 45 00 10 08 31 00 00 08 32 00 08 08 4C 00 10 10 33 00 00 10 34 00 08 10 35 00 10 18 36 00 00 18 6B 00 08 18 37 00 10 20 83 00 00 20 84 00 08 20 85 00 10
Plus an Odie loss image.
00 BD 03 00 00 BE 03 08 00 BF 03 10 08 CD 03 00 08 CE 03 08 08 CF 03 10 10 DD 03 00 10 DE 03 08 18 DF 03 10 18 ED 03 00 18 EE 03 08 20 EF 03 10 20 EA 03 00 20 EB 03 08 28 FB 03 08 1A FA 03 18
I also wanted to add two sprites to this menu screen, for minor fixes.

Here's Odie's three running frames, which would cycle 1232 during gameplay, and pause on a win.
00 93 03 00 00 94 03 08 00 A3 03 10 08 3D 03 00 08 3E 03 08 08 3F 03 10 10 4F 03 10 10 4E 03 08 18 5F 03 10 18 5D 03 00 18 5E 03 08 20 6F 03 10 20 6D 03 00 20 6E 03 08 28 7F 03 10 28 7D 03 00 28 7E 03 08 20 EC 03 18 28 D8 03 18 08 D4 03 18 10 E4 03 18 18 D9 03 18
10 D7 03 00 08 C7 03 00 08 C8 03 08 08 C9 03 10 10 4F 03 10 10 4E 03 08 18 E9 03 10 18 E7 03 00 18 E8 03 08 20 F9 03 10 20 F7 03 00 20 F8 03 08 28 7C 03 10 28 7A 03 00 28 7B 03 08 08 D4 03 18 10 E4 03 18
00 93 03 00 00 94 03 08 00 A3 03 10 08 3D 03 00 08 3E 03 08 08 3F 03 10 10 4F 03 10 10 4E 03 08 18 5F 03 10 18 5D 03 00 18 5E 03 08 20 E3 03 10 20 B3 03 00 20 6E 03 08 28 F3 03 10 28 C3 03 00 28 C4 03 08 28 F4 03 18 08 D4 03 18 10 E4 03 18 18 D9 03 18