screen shot does not work

i am using SuSE 8.2 with KDE. i try to capture the screen using the keys ALT+PRINT or CTRL+PRINT, nothing happen. the clipboard is alway empty. i checked that the use key sequence are active in the control center (general key settings). Are there other setting that are needed to capture a screen or is there an application that i have to install before i can use ALT+PRINT on KDE? any help are welcome. Theo