flash video? you mean flv or swf? just press print screen button on your keyboard and paste it to an image editing software
by scree shooting u can have only one frame of any flash work. I think when u wanna capture flash work from any website u must want to get the whole work which contains many frames, sometimes sounds. so U can use any "flash decompiler" tool and capture it. then u can modify it. thas just kool. search in google with the keay words " Flash decompiler" for good ones u may pay.