You can do it in PS or most any other graphic program. You just make the signature with a clear background and then paste it on the corner of the picture. You can adjust for size and color and what not with each picture.
Actually you can do it with any basic program, even with programs that are automatically installed in your computer. (If they have a text creator in it)
Just make sure you don't save your picture as a 24-bit Bitmap afterwards because the text will normally turn out bad.