ALT-S will take a shot and put a file in your main FA-18 folder called DUMP00x.bmp where the x will increment as more shots are taken.(DUMP001.bmp, DUMP002.bmp etc..).
Alternatively, you may wish to use a 3rd party application such as HYPERSNAP.
You will then have to crop/resize the image to fit in the BRIEFING and OBJECTIVE boxes.
Dimensions are given in the Mission Builder FAQ section of this site.