I have noticed that my thumbnails have a purple outline on them when viewing in Safari. They do not appear in Firefox or Chrome so I am hoping there is some CSS to hide them from diplay in Safari?
link: http://charliescookies.com.au/dev/wholesale-brochures/
It seems you are using a 3rd party plugins to generate those images. The blue border is the background color of those images. You can remove it using following CSS code under Custom > CSS in the Customizer: