Create Separate Thumbnail image for Portfolio Item using Lightbox

[cmd-developer]

Hello,
How can I use a separate image as the thumbnail in the Unicom Portfolio that then uses the Featured image as the image that opens in Lightbox?

E.g. the ability to set a specific thumnbanil image to be shown on the Portfolio page and when clicked it will open the full size featured image in Lightbox?

I would like to do this as the Portfolio icon overlay/ default setting that I am using crops the images to all be the same size on the page (the thumbnails are cropped)

Thank you for your help!!

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