When sharing your marketing piece on facebook, you'll need to keep in mind the minimum image size that facebook requires.
They recommend the image be at least 600 x 315 pixels, and anything smaller will appear smaller in the actual post.
That said, there is a hard minimum that they will not accept. Anything under 200 x 200 pixels will be rejected and not shown in the end result of the post.
For more in depth reading, see here.