Which permission for pictures? 644? In order to avoid some non wanted modifications or corruption of pictures files, one possibility is to change the permissions of the files (in ~/Pictures). 

Do you think that this idea is a good one? And which permissions? 644 on files and 755 on directories?
0644 is good for pictures. 0755 is only needed for executable files and directories (thanks,&nbsp;Mateusz).

Note that 0644 grants read access to all users. If you want to deny read access to other users, then you might consider 0600 instead.

More info: man 1 chmod.