Hey,
sorry for not replying any sooner. You could try to regenerate thumbnails with the Regenerate Thumbnails plugin but please make a backup before you do this (better to be safe than sorry).
Now, there is another option but it is a bit complicated:
First, go to your create-new-post page or — even better — in one of your previous already-published-posts. On the right top corner you should see the ‘Screen Options’ button; click on it for the drop-down menu to open. After that, tick the box next to ‘Custom Fields’.
Now you should be able to see a new box below the editor called ‘Custom Fields’. Create new custom field, name your image that you would later like to use it as a thumbnail for this specific post (something like “My thumbnail”), enter the URL of the image you would like to use and finish with the click on the button “Add Custom Field”. Of course, don’t forget to save this draft, or publish the post when you’re ready.
After that, visit the settings of (y)our plugin and scroll down to the bottom. The option called “Use custom field for thumbnails” should now be available, tick the box next to it, and select the value you’ve set up before (if you used “My thumbnail”, select that one).
Now, the important thing is this: the value of the Custom Field stays the same for every new post, while the URL of the image (obviously) changes. In other words: the name of the Custom Field stays the same, images change. That’s because once you set up your settings in the plugin settings (when you choose which value our the Custom Field our plugin should use for the thumbnails), you won’t have to change it again in the future. This way you save yourself a lot of time.
Once you have set this up, you just add a Custom Field with every new post, set the image you want to use for the thumbnail, while the name stays the same.
Let me know how it goes,
Petra
