The minimum allowed image dimension is 200 x 200 pixels.
The size of the image file must not exceed 8 MB.
Use images that are at least 1200 x 630 pixels for the best display on high resolution devices. At the minimum, you should use images that are 600 x 315 pixels to display link page posts with larger images.
If your image is smaller than 600 x 315 px, it will still display in the link page post, but the size will be much smaller.
We've also re-designed link page posts so that the aspect ratio for images is the same across desktop and mobile Feed. Try to keep your images as close to 1.91:1 aspect ratio as possible to display the full image in Feed without any cropping.