Images are optional. You can create staff directory with no images.
You can use the same picture on staff and single pages or have two different ones.
Page layouts and images.
List, Grid A, Grid B and Single Pages can have staff images.
Grid C has no no option to add pictures.
How to add staff images.
Staff Member > Staff Member Data > Images.
How to make images of the same size.
In responsive layouts (like Staff List), images are resized to fit the container. The maximum image size is 100% of the original. Images are not upsized or cropped.
To show images uniform in size, they need to have the same dimensions or aspect ratio.
You can use WordPress image editor or any other graphic application for that purpose.
Free, online image editors: ResizeImage.net or google “online image resizer”.
Staff List plugin doesn’t crop, resize, compress or change the image quality. Images are loaded as is. They may be scaled down by the browser but won’t be scaled up.
Large, high-resolution or uncompressed images are slowing down your page load time. That may lead to some issues, especially in case of low bandwidth or mobile devices.
Image optimization is about reducing the file size (not image dimensions) as much as possible without sacrificing quality. With smaller files, your page load times remain low. You may want to google How to optimize images for web for more info.
Most modern browsers implement native lazy load option, which can significantly improve page loading time. It usually requires adding loading attribute to an image tag.