Low resolution in Debian

I installed Debian 8.3 LXDE today and I cannot adjust the resolution of my monitor. I have a 1920x1080 resolution, but allows you to install Debian to me only 1280x1024. I tried to make as here: https://wiki.archlinux.org/index.php/xrandr , using xrandr to do manually, but there is no connected interfaces: drahenfels@debian:~$ xrandr xrandr: Failed to get size of gamma for output default Screen 0: minimum 640 x 400, current 1280 x 1024, maximum 1280 x 1024 default connected 1280x1024+0+0 0mm x 0mm 1280x1024 0.00* 1152x864 0.00 1024x768 0.00 800x600 0.00 640x480 0.00 720x400 0.00 My GPU: AMD/ATI RS780L [Radeon 3000]