A glance at the demo at: https://demo.getpublii.eu/themes/persona/violet/ for example, uses source images with a width of 1600px and whatever proportional height each image requires. So, perhaps try to mimic the 1600px width.
To allow for the various image sizes that the Persona theme will generate from the source image, perhaps for most of your images, you may want to have the centre of attention in the image to be as near to the middle of the image.
Hope that helps.
