If you choose to delete Placid for WordPress, all images that you created with Placid will be removed from your blog, and you will not be able to generate images for your new posts.
Which images will then be displayed on Social Media depends on your WordPress theme, and additional plugins. If one of your posts with a generated Placid-image was shared a lot, it can happen that this image still appears when it's shared. Some platforms do not change the image any more if the post already got a lot of shares.
If you later want to reinstall Placid, your templates will be available again once you enter your license key.