I've followed the advice given in this post. I have updated the Facebook cache, and shared a website on Facebook on my Facebook timeline (shared only with me for testing purposes) via a web browser. When I do this, the image in the sharer looks fine, and when I go to my Facebook page I can see the shared link details as well as its corresponding image, which is a 1200x630 PNG (the text is displayed immediately but the image takes about 6 seconds to load, despite the fact that I thought such image would have been stored in the FB cache).
The story is different on my Facebook for Android app. I had previously shared a website whose site image is a 1200x630 PNG, and I can still see that image on my FB Timeline despite the fact that it's been removed from my site a few days ago and replaced with another image. The problem is that the newer og:image I've updated on my website, which is the same size and format as the older one it replaces, displays fine on my mobile phone's web browser and on my PC's web browser, but does not show up in my Facebook app for Android, and a blank ligh-gray rectangule with the shared og:title and my home computer's IP but no og:image shows up in the shared post. A whole day has gone past since I've updated the FB cache, and I've tried the Facebook Debugger Tool once again, but the image just won't show.
How do I fix this?