Image capture using v4l2 utils

Hi,
I am using v4lt utils to capture image using an embedded device. I can able to capture image of resolution 320x240 and 64x480 properly. But when I increase the resolution more than this then the streaming gets slower and its not possible to set frame in particular display size but the image is captured in the specified resolution. Kindly assist me to set the screen size(320x430) to display the live streaming before capture and to capture the image in specified resolution(1280x1024). I am using pixel format as MJPEG. I need to capture the same image that is displayed in the streaming window.