In the window to write the message at the top in the middle there is a picture icon ("Insert Image"). Next, specify the path to the screenshot (if you used the standard F8 in the game, the path is something like this: C:\Users\Ivan\Documents\GTA San Andreas User Files\SAMP\screens\sa-mp-***
Ivan - the name of your account in the PC.
But it's really only for Windows, for other OS's idk.
Regards.