Copy and paste the following into the Script Editor, click the hammer icon (compile), and then the Run button. You can find Script Editor in Dock : Launchpad : Other.
-- prompt for parent folder of images, and then rename each image to the following
-- format: nnnnnn-nnnnnnC.jpg from original nnnnnn.jpg
-- Tested: macOS Mojave 10.14.6 (18G6020) and macOS Catalina 10.15.6 (19G73)
-- vikingOSX, 2020-07-23, Apple Support Communities, No warranties express/implied.
use scripting additions
property images : {"jpg", "JPG", "jpeg", "JPEG"}
property suffix : "C"
set sfolder to (choose folder default location (path to desktop))
tell application "Finder"
set imglist to (every item in folder sfolder whose name extension is in images) as alias list
if imglist is {} then return
-- strip dot and file extension from the filename
set {TID, AppleScript's text item delimiters} to {AppleScript's text item delimiters, {".", images}}
repeat with anImg in imglist
set tmp to item 1 of (name of anImg as text)
set newname to tmp & "-" & tmp & suffix & "." & name extension of anImg
set name of anImg to newname
end repeat
set AppleScript's text item delimiters to TID
end tell
return