Calvin: I tried to run your post the the FB debugger:
https://developers.facebook.com/tools/debug/og/object?q=http%3A%2F%2Fdmzstory.org%2Fwebsite-instructions-en%2F
This gave me an error.
So, I tried to run your root domain through the bugger:
https://developers.facebook.com/tools/debug/og/object?q=http%3A%2F%2Fdmzstory.org%2F
Which gave me other errors that maybe you could fix. It grabbed the image from the first image that it could find on the page, as far as I can tell. If you include a default og:image in your theme, it’ll use that at least instead of the Gmail logo.
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I did the same for your site:
https://developers.facebook.com/tools/debug/og/object?q=http%3A%2F%2Fwaynemckenzie.com%2Fkelli-leigh-missing-you%2F
and it notes that it can’t download that image with the truncated file name.
So, there’s no reason for Jetpack or WordPress to be truncating that filename. Are you uploading the image with the -300×200 already included? Can you check in your media library for it? because it almost appears to me like some function in your site is causing this, but without being able to see your site’s code, I can’t be certain.