The command you are looking for is:
find . -type f \( -name '*.jpg' -or -name '*.png' \) -not -name "cover.*"
type -f will make the
find command look only for files.
In your second command, you need to add a space after
\( and before
\) (you also forgot
Also, you don't need a
-name. Your first command works fine if you remove it, although not producing the output that you want (see JoL's comment).
You can read more about
find's syntax and options at the command's online Ubuntu manpage, or run the command
man find to read the manual in the terminal. Specifically, read the
OPERATORS section of the manual, since that's what appears to be confusing you the most.
Note: Most terminal commands provide a manual with the proper command syntax and available options which you can read if you run
man <command> in your terminal.