If you have a file management app, then I recommend you access it and browse the internal storage. WeChat (just like any app of this kind) keeps the images and every other media file received or sent in folders based on file type. Go to internal storage and look for the WeChat folder. Tap on it, and you will see other folders arranged based on their type. Locate the image folder and delete the files from there. When you tap that option, WeChat copies the already stored files in the Gallery's folder (eg. DCIM or Pictures).