Where did you copy them? Somewhere in your home directory? Or to the root of the file system? The latter could be troubling.
It just be that the ownership or the permissions on the files is goofed up.
This would be easy to make quick work of if you know the 'chown' command, because you could open a termina window and type something like:
chown -R <yourusername>:staff
And then before typing return, drag the folder with all the files on top of it.
However if you haven't used the terminal there's a chance of getting it wrong. <yourusername> is short for whatver your UNIX username is... also known as your user short name.
The 300 jpegs were on cd in three separate folders. I originally copied them to my computer in their folders and that's when the problems began. I managed to delete the folders and started again, this time just copying the files (not their enclosing folders) and they are behaving fine.
Oddly enough the permissions still say Read only but I can delete them and change their file names without needing to use a system password.