Can't Rename Images Sequentially Via Applescript Because Other Images Already Have That Name
The main problem is similar to your other topic - the new name + suffix is not checked against what is already in the folder.  My answer to that topic used a general-purpose handler for getting a new name, so without using that, the (stripped down) contents are used instead:

set all_files to (choose file with prompt "Choose the Files you'd like to rename:" with multiple selections allowed)
set theName to "Hello World " -- includes separator text

tell application "Finder"
    set theFolder to container of first item of all_files -- the base folder
    repeat with this_file in all_files
        set theExtension to name extension of this_file -- keep existing extension
        if theExtension is not "" then set theExtension to "." &amp; theExtension
        if name of this_file does not start with theName then -- don't rename renamed items

            # the "increment suffix number until the name is unique" part
            set counter to 1 -- starting suffix
            set newName to theName &amp; counter &amp; theExtension
            tell (get name of items of theFolder) to repeat while it contains newName -- get new name
                set counter to counter + 1
                set newName to theName &amp; counter &amp; theExtension
            end repeat

            set name of this_file to newName
        end if
    end repeat
end tell