if i double click on a JPG in Ubuntu, it opens up whatever picture viewer is default, just like windows. i don't believe executable exists in Linux--everything is a file!
Now our college used and we were required to have our own version (trial) of WinNT 4 and Win2K and i do know the Guest account cannot even set the system time! much less change sensitive info. a 'user' in Linux (i do not recall there being sub-groups in *nix such as guest, power user, and so on just user and root) has more power. a 'user' can use his own user password (Ubuntu does not set a separate root password in its installer ) to sudo anything. there is no security when a person can simply use a generic user password to gain even temporary root priviledge, which can be dangerous in itself. but i don't see too many Linux users complaining about problems the way Windows does.
Samsung Galaxy Precedent (rooted, AreaRom 3.4 , sd-ext w/ 3.65GB internal space)
Kindle Fire (1st Gen, rooted, AOKP JB 4.1, Nova Launcher)
Samsung Galaxy SIII 4G LTE (rooted, Carbon ROM)