Random Numbers/Letters

Hello,

    This may have been discussed before(I have looked and could not find
much on this), but I need to find a way to generate random numbers and
letters together to insert into a field when a new entry is created.  Just
to give you a little insight, I will be accepting images from people and I
want to generate a random line of numbers or letters so that I can put a
name on a picture, and when someone pulls up a profile of the user, it will
pull the picture with the random name out of the directory.  I haven't been
able to find much on random number/letter generators within PostgreSQL so I
wanted to post here.  If anyone has an easy way for this to be done, please
let me know.  Thanks :)

Preston