I don’t know much about 7800 graphics so I started reverse engineering them in Hack-o-Matic III and Tile 'M
So far this is what I’ve discovered. Jr. Pac-man has a couple banks of sprites starting at 0x0 and 0x6000 both of length 0xF00. Each sprite is read left to right and rows are separated by 0x100 (256) bytes. Each Pixel is 2 bits so the first 2 bytes represents the first row. The 2 bit value determines which color in the palette is used. (00 = color0, 01 = color 1, 10 = color 2, 11 = color 3). Here is how you would read the first sprite in the second bank of graphics.
Row 0 [0x6000 -0x6001] Row 1 [0x6100 -0x6101] Row 2 [0x6200 -0x6201] Row 3 [0x6300 -0x6301] Row 4 [0x6400 -0x6401] Row 5 [0x6500 -0x6501] Row 6 [0x6600 -0x6601] Row 7 [0x6700 -0x6701] Row 8 [0x6800 -0x6801] Row 9 [0x6900 -0x6901] Row 10 [0x6A00 -0x6A01] Row 11 [0x6B00 -0x6B01] Row 12 [0x6C00 -0x6C01] - Always Blank Row 13 [0x6D00 -0x6D01] - Always Blank Row 14 [0x6E00 -0x6E01] - Always Blank Row 15 [0x6F00 -0x6F01] - Always Blank
Since this will result in an image that is upside down it should be flipped for easy viewing.
All the other games in the Pac-man series store sprite data differently. Data is also read left to right, with the exception that rows are sequential so you can read in a sequential set of 32 bytes. Colors work the same. The only problem with this is that graphics wrap so sometimes you need to edit 2 sprites to really edit the image.
Row 0 [0x6000 -0x6001] Row 1 [0x6002 -0x6003] Row 2 [0x6004 -0x6005] Row 3 [0x6006 -0x6007] Row 4 [0x6008 -0x6009] Row 5 [0x600A -0x600B] Row 6 [0x600C -0x600D] Row 7 [0x600E -0x600F] Row 8 [0x6010 -0x6011] Row 9 [0x6012 -0x6013] Row 10 [0x6014 -0x6015] Row 11 [0x6016 -0x6017] Row 12 [0x6018 -0x6019] - Always Blank Row 13 [0x601A -0x601B] - Always Blank Row 14 [0x601C-0x601D] - Always Blank Row 15 [0x601E -0x601F] - Always Blank