It's not possible to change the thumbnail displayed for your images.
The thumbnails on your profile are currently fixed to a centred square crop of your image, regardless of whether the original image is in portrait or landscape format. On Picfair Stores, thumbnails will be displayed in the image's original aspect ratio. The only exception to this is the square display format available on Picfair Plus Stores, where a centred crop of the image will apply.
We're always looking for ways to improve how your images are displayed, and we'll keep you updated as we develop new display features.