If you want to replace the default background of the Messenger card in your Abandoned Cart or Instant Messenger campaigns, the ideal resolution is 1000 x 520px. If you wish to use a different resolution, please make sure, you keep the image ratio at 1 : 1.91.
The 1000 x 520px width is recommended because of the large, high-resolution devices. If you only have a 500px wide picture for background, that's fine, the users will hardly notice the difference.
Don't use light background in Abandoned Cart Campaign
In order to keep the text readable, Facebook puts a 40% transparent black overlay on your image. Nevertheless, we recommend that you avoid white or light backgrounds, as the text might still be difficult to read on them.
Don't do this:
Choose a darker background instead:
I hope this helps. If you have any questions, please contact us on the support chat.