Geni does a great job of resizing photos or you, but photos with an aspect ratio of 4x3 are ideal. Some common examples of 4x3 aspect ratio are 640x480, 1024x768, 1600x1200, and 2048x1536. If the photo you are adding to Geni is going to be the main photo for the profile, try to make sure that the person's face is in the center of the photo so that their face shows up on their box in the tree.
Geni stores images at a maximum resolution of 640x480. So for more control over the quality of your image you can resize it before uploading to no larger than 640x480.