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.
Geni accepts the following file formats: jpeg, png, and gif.
'Tagging' a photo is a way to indicate who is in a photo on Geni. You can tag a photo with a person's name by clicking the 'edit' link next to 'In this photo' under any photo. You can choose a name from the pop-up list or start typing the name of a family member or a family friend. When you have added the name of each person in the photo click done.
This photo will now be associated with each person you tagged. If this is the first photo tagged with a person's name this will be their profile photo.
Related link: How to Upload Photos on Geni.com (YouTube video)