I hesitate to explain because large photos in the signature area are a bit of an annoyance. However, on the chance you wish to put a small photo there, then all you need is the photograph to be on a web-accessible server so that you can get the url for the photo. Then, in your signature area, past the url surrounded by the html code for displaying images. That code entails using the brackets [ and ] with the terms img inside, followed by the url of the image and closed off with /img within the brackets.
ignoring the _ character, the following is the form:
[_img]http://picture.com/picture.jpg[_/img]