Get the best chance for the best outputs with choosing the best input images. Any images are not bad, but if you want the best outputs, here are some tips to help!
Arms and legs are separated from the main body. Hands or arms not touching body.
In a standard pose, a-pose, t-pose, facing straight on. (This is best if you plan on rigging the character, if not any pose is fine)
No clothes or weapons hiding information on the character.
Close up portraits
Weird poses (If you plan on rigging character, if not this is okay)
Weapons or clothing obstructing information on the character
Busy backgrounds or character blends into background