Take a screenshot using F8, then browse to your GTA San Andreas User Files folder which should be in
C:\Users\USER\Documents\GTA San Andreas User Files Go into the "SAMP" folder and then the "Screens" folder.
C:\Users\USER\Documents\GTA San Andreas User Files\SAMP\screens default. All of your screenshots should be saved in here. To upload them on to the forum I suggest you make an account on
photobucket and upload your pictures to this website. You will then be given a IMG link which you can post on forums which will display your picture. The link should look something like:
[IMG]http://image url[/img]
.