The most common reason for this is that an add-on or extension you have installed in your web browser (often Firefox) is blocking the photos being loaded from Facebook. Try checking to see whether you have any add-ons or extensions installed in your browser and, if so, try disabling them to see whether that solves the problem.
Another possible reason for images not being displayed is if you’re browsing using Firefox’s “Private Browsing” mode and have “Tracking Protection” enabled in the settings. You would need to disable this as it blocks some Facebook images.
You can check whether the issue is related to something in the web browser that you’re using by just viewing the site in a different browser. If the images display correctly in a different browser then it indicates that something in your specific web browser is preventing the images from loading.