How can insert a picture into a sketch at full scale?

There is no way to "set" a default insertion size for an image to be 1:1. because of the nature of pictures, and that they can vary in many ways such as resolution and size, solidworks has made it a standard that the picture is inserted with its (0, 0) coordinate located at the sketch origin, and additionally, the initial size of picture is determinied by the number of pixels. SWX uses the covention of 1 pixel/mm, and locks the aspect ratio. At this point, if you know the original size of the picture, and the number of pixels, you can figure out what scale factor you should use as a starting point to get the sizing in the ball park (you can use any imaging software to find these values, even Paint). Use sketch lines, and the measure to to fine tune it if you have some geometry of known size in the picture. (thats ALWAYS A PLUS)