Hello. I've seen this error occur an X amount of times and I've answered it quite a lot myself so I thought I'd post it.
Example:
Error: Unable to save screenshot.
This is a very common error for players that use Windows 8. I've experienced it myself and I couldn't find a solution for it, so I started using Fraps to take screenshots, but that was irritating because it took more time, untill I decided to put my resolution higher because I wanted better graphics, and suddenly I was able to take screenshots.
The cause of this error is the resolution you're playing GTA with. To fix it, simply follow these steps.
Step one. Open up GTA San Andreas.
Step two. When you're in game, press ESC
Step three. Press "Settings"
Step four. Press "Advanced"
Step five. Go to "Resolution" by clicking on it or stand on it with your mouse curser.
Step six. Change your resolution, upwards or downwards doesn't matter.
You are now able to take screenshots again by the use of F8.