to import pictures you go to file>open... and that will allow you to browse your desktop and find the image you want.
you can always change the size of any image by going to image>image size.. from there you can resize it and keep the proportions or you may change the size and proportions. Example, change a 200x200 to a 200x100, if you wish.
Once you get an image opened, you can put it in a layer of your sig by cutting it out wih a pen, or lasso, or eraser, and making that a selecion with the selecion tool (black arrow on the left column) and cut it out of the image by dragging it over into your sig.
"When I know nothing, the possibilities are endless."