If you are talking about taking a screenie with ALT_S - it dumps a ahss##.bmp file in you game installement (HTC/Aces High II) folder. A whole bunchload of programs will open it.... just try a double click.
If youre talking about a Screenshot taken with PrintScreen - ALT_Printscreen takes a screenshot of the current active window (to get one of the whole screen just press Printscreen). The data gets temporarily stored on your Clipboard. After pressing the key, just open some graphic editing software - MS Paint, Photoshop, GIMP, ... , open a new file and paste the content of the clipboard into the file (CTRL_V or "paste" in the menu). Then just "save as".