1. A method of estimating a blood pressure based on an image, comprising: obtaining, by camera, an image including a skin of a user; determining, by a computer device, a skin region, in which at least one portion of the skin is displayed, from the image, and storing a mean value of color data of a designated color model for each of two target regions which have different positions in the skin region from each other; estimating, by the computer device, a pulse-wave transit time (PTT) based on a pulse wave signal determined based on a change in the mean value of the color data for each of the two target regions; and estimating, by the computer device, the blood pressure using the PTT.