Mobile Banners linked though website constantly disappearing

Site URL: http://www.georgeprice.co.uk

Hello,

I need help with mobile banners on my page, I use some custom CSS that I found though an online tutorial where I open up the banner images I want to use for mobile each in there separate browser tab (by loading the images onto one of my pages right clicking and pressing open in new tab which just loads the image as its own tab with a long Squarespace http link) I then copy this link into my custom CSS for each page, and this works perfectly all my mobile images are available and resize to different mobile screens, however when I periodically check my square space site the mobile image sections are often blank and I have to go though the whole process again of opening all my mobile images as tabs and copying the link into my custom CSS. I was wondering if anyone had a solution to this as my site is my online portfolio and I don't want to have to constantly worry if my site is working properly on mobile or not.