Press "Print Screen" button on your keyboard to take a screenshot, if taken ingame they are saved in the Wc3 folder which should contain a folder called Screenshots or something..
If they are taken within the editor you need to press print screen and then open an image program like mspaint,gimp,psp or photoshop and paste by pressing "ctrl+v".
and to import them, you need to resize the image to the dimensions of 64, eg:
16
32
64
128
256
512
1024
2048
but you can have it like:
512x1024 or such or the other way around.
and what do you need to import them for anyway?