When you do this: "LOAD BITMAP "F15.bmp",1" you tell Darkbasic to load that bitmap to screen 1. Screen zero is the main screen. Anytime you load a bitmap on a different screen other than zero it changes the current working screen to the number you use (in this case screen 1). You need to change the current working screen back to zero before you can see anything. To change it use "set current bitmap 0" after getting the image.
LOAD BITMAP "F15.bmp",1
GET IMAGE 1,0,0,300,300
set current bitmap 0
SPRITE 1,170,90,1
WAIT KEY
Also you can save from having to change bitmaps if you use "load image" instead of "load bitmap". "load image" is both "load bitmap" and "get image" in one command. It's mainly used for images that only have one graphic you want (load bitmaps to grab multiple images on the same picture).
load image "F15.bmp",1,1
SPRITE 1,170,90,1
WAIT KEY