Hi Lucian,
I tried with jpg as well as png images but it is not working on Whatsapp.
In fact, I tried with an image from your sample blog post, that also did not work.
Additional info: I edited the Open Item Collection Template and removed the element of Image which it was picked from the Image field (as shown in the attached screen grab) as I just wanted to show the title of the blog. Could it be a reason?