height of each frame needs to be multiple of 8

If you use an image (with or without frames), each frame height has to be an multiple of 8.
This means that if your frame is actually 6 pixels high, you'll have to add 2 empty pixels.
so if the image has 3 frames of 5 pixels high, your image needs to have a total height of 24 and not 15.

The resulting array needs to be in full bytes, so multiple of 8

img2ard could take this in account, by adding the missing bits by itself

if you name the image correct: image_5x6.png img2ard should now the height is 6 but it needs to ad 2 extra empty pixels for the output.

[joshgoebel]

Please provide a sample PNG and the output you're seeing.