Install Webcam driver: 

http://www.ibuyopenwrt.com/index.php/8-yun-compatible/60-uvc-webcam

http://www.ibuyopenwrt.com/index.php/8-yun-compatible/61-non-uvc-camera-gspca-camera

Install fswebcam:

opkg update
opkg install fswebcam

Make sure driver installed:

root@Arduino:/mnt/sda1# ls /dev/video0  -al
crw-r--r--    1 root     root       81,   0 Jul  7 07:08 /dev/video0
root@Arduino:/mnt/sda1# fswebcam -d v4l2:/dev/video0 -i 0 --list-controls
--- Opening v4l2:/dev/video0...
/dev/video0 opened.
Available Controls        Current Value   Range
------------------        -------------   -----
Brightness                127 (49%)       0 - 255
Contrast                  32 (12%)        0 - 255
Saturation                32 (12%)        0 - 255
White Balance Temperature, Auto True            True | False
Gamma                     220 (100%)      100 - 220
Gain                      0 (0%)          0 - 255
Power Line Frequency      60 Hz           Disabled | 50 Hz | 60 Hz
White Balance Temperature 4000 (32%)      2800 - 6500
Sharpness                 180 (70%)       0 - 255
Backlight Compensation    1               0 - 2
Adjusting resolution from 384x288 to 352x288.
--- Capturing frame...
Captured frame in 0.00 seconds.
--- Processing captured image...
There are unsaved changes to the image.