Image sizing for the desktop news feed varies. Since the recommended ad image sizes are 1200 pixels wide, the same can be applied to images in posts to your profile or page. You can go as small as 470 pixels since that’s the size that ad images are resized to in the desktop news feed. However, note that when someone clicks on the image, it won’t take up the full space of the lightbox.
For example, this is how a 1200 pixel wide by 1200 pixel high image looks when posted as a photo to your page.
The recommended image size for ads with photo posts is 1,200 pixels wide by 900 pixels high, which looks like this when posted to your page.