To edit a card not necessary click icon edit, whole card is edit trigger. Icon marked just you know that you can edit card
You can use external images as well. For this you no need to use media library, just enter its URL in the field(see screenshot)
Remember - Some can have copyright, use your own from another sites or images under necessary licences
You can use widget to display your Cards as well.
Use it if your theme has widgets in the necessary areas.
Go to widgets find OSS Cards Widget, add and select in dropdown.
This method is useful if you want to use cards outside content area, for example in sidebar, footer etc
To look balanced the script makes same height of cards in row. It makes same only content(description) container, because description often can be different length, images always has same height, linls should be short
Avoid the couple of things - do links in one line and do not have cards with link and without in same row.