Control cropping of preview image – Actual image in Featured image

Hi Team Kriesi!

I have about 80 images to display (all in the same category) so I’ve decided to go with a full-width 4 column portfolio grid for my Portfolio page.

I’m trying to create a portfolio grid that opens up the actual image using the ‘Display the big image in a lightbox’ option. There is no standard size for the actual image to be displayed on click, although I’m trying to make sure they’re all bigger than 700×700.

My question is this: The actual image should be set in the ‘Featured Image’, right? How do I control the cropping of the preview in the 4-column grid?

In future, I would also like to be able to manually create thumbnails, i.e possibly have black & white images with the client logo watermarked on the preview image. How do I go about achieving this?

Thanks much!