The best pictures are shot in natural light. Avoid pictures shot indoors, using a flash, if the light is not perfect.
Avoid strongly contrasted light on faces, with part of the face in the shade. The whole face must be well lit.
The colors in the photo-to-chart conversions by Amelie Paris will be exactly those on the original picture.
Carefully analyze the real colors of the pictures, not the “memory” you retained of the original colors. Colors might have lost their brightness in the photograph because of the lighting or unfortunate shades. A white wall could turn out grey or beige on the picture and that will also be the color it will be stitched in.
Focusing on the subject:
Pick a photo where the main subject occupies most of the space of the picture and stands out against the background (wall, sky, beach…).