Permissions & Administrator ?s

Hey,
I have been trying to get my iMac to stop asking for a password every time I try to save a photo or move a file whatever! I AM the administrator. No one but me uses this machine. Which btw is Mac OSX version 10.6.7
Processor 2.16 Ghz Intel Core 2 Duo
Memory 2 GB 667 Mhz DDR2 SDRAM
I want to delete all other accounts (this was a used iMac) and have only my account. Sometimes I get a dialog box that says I din't have permissions. What up?
Also, how does one go about deleting disk images .dmg? The previous owner has one I don't want and when I restart, there it is again and I trash it.
Mike

It's always best when purchasing a used computer to completely erase the hard drive and do a reinstall. Did the previous owner include the original DVD set that came with the machine? If not, you can do the same thing with a retail version of OS X. (You say you have 10.6.7.)