1. A method implemented by a computing device to define a portion of an image as selected based on selection input, the method comprising: receiving a selection input by the computing device for selecting a portion of an image; determining probabilities by the computing device that pixels of the image that correspond to the selection input are intended to be included as part of a selected portion of the image, the probability for a pixel determined as a function of position relative to center pixels of the selection input and a difference in one or more visual characteristics with the center pixels; applying a segmentation technique to the image by the computing device to segment the selected portion from a remainder of the image, the segmentation technique identifying the pixels of the image as part of the selected portion or as part of the remainder of the image based, in part, on the probabilities; and generating selected portion data by the computing device that defines the selected portion according to the segmentation of the selected portion from the remainder of the image.