The Nintendo 64 supports, by default, a few different image formats. Every image format has its own pros and cons, and these pages should help you understand when to choose one over another. Each page builds on knowledge from the previous ones, so it is recommended that you read them in order.
Information on the different image formats supported by the Nintendo 64 can be found in the following pages:
RGBA
Color Index
Intensity
Intensity With Alpha
YUV