The facebook debugger tool will usually fix this right away. Basically, facebook needs to "scrape" your post again in order to recognize the latest pic.
In this video, Mariah from Mariah Magazine Web & Strategy Studio does an excellent job of explaining why this happens, and walks you through how to use the debugger tool. It's a 3:18 minute video and well worth the watch.
If you don't care to understand why facebook does this and just want instruction on the tool, fast forward to minute 1:18.

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