When I load an 800*600 image as a room background and I test the game, the image appears thinner than it is. but when I take an screenshot, the screenshot is perfect. Is it an AGS bug, or is it my pc?
It's the resolution settings of your monitor. When you're watching the screenshot of the game, you presumably do so in Windows' natural resolution. When you run the game (in full-screen), it's in 800x600. You probably have adjusted your display manually using the buttons on the monitor, so it's set up properly only for your standard Windows resolution. Most monitors have calibration software that allows you to run through all possible display settings and adjust how they show up on-screen and store these values.
Try running the game windowed, my guess would be that it looks perfect there. If so, have fun calibrating your screen :)
Thank you, very much. I will try it. ;)