Zkontroluj si ze mas v /etc/apt/sources.list (misto jessie muze byt wheezy, zalezi jakou mas verzi debianu, ale i tak doporucuji upgrade na jessie):
deb http://ftp.cz.debian.org/debian/ jessie main
deb-src http://ftp.cz.debian.org/debian/ jessie main
deb http://security.debian.org/ jessie/updates main
deb-src http://security.debian.org/ jessie/updates main
# jessie-updates, previously known as 'volatile'
deb http://ftp.cz.debian.org/debian/ jessie-updates main
deb-src http://ftp.cz.debian.org/debian/ jessie-updates main
pak uz jen:
apt-get update
apt-get upgrade
apt-get dist-upgrage
apt-get autoremove
apt-get install gimp