Hello
Have someone idea how I can take snapshot from some camera when we have signal from GV-NET/IO card input.

Thank you very much in advance

Well, you only need a snap shot if you are going to email it to somewhere. There isn't much point in setting a snap shot aside if you also have the video.

So, presuming you want that snap shot to go somewhere:

General Settings->System Configure.
On the I/O device tab, pick the input you want to monitor, select "invoke to send alerts" and click on the green arrow to the right of that option. You should get a pop up with a list of cameras. Select the cameras you want to include in the alert.

You also need to have the I/O activated in the scheduler.
In the email setup you have to have "attach image setup" selected.

I need this snap shot because it is a image of car licence plate then I use this snap shot to make a html table with this info.

You might be able to write a program that grabs the still JPG images that GEO creates when you launch the GEO webserver with the JPEG option checked. If you had a program and an external input - perhaps separate from the GVIO, you could have this trigger and save the current jpeg image that GEO has saved on the hard drive.