ISSUE ADDING ICONS TO CUSTOMISE SECTION

I purchased a pre-made WordPress design theme and changed the icons on the pre-made theme. I am trying to replace the icons with ones of my own but the links i’m inputting are not being accepted? and if they do they’re pushing the theme out of place? I want to simply add my own icon options?

how do I do this. I’ve added a link of screenshots. You can see how they’re suppose to look.

The page I need help with: [log in to see the link]