Hey Andre,
thanks for reaching out – if I am not mistaken you have also contacted us via email but we can continue here if you wish.
First thing first, may I get a link to your blog, so that I can take a closer look?
As far as those custom fields go, there are some things to consider:
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 be ticked, 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. Also worth noting is that you should keep the images you want to use for thumbnails as square as possible and that they’re not too big. The perfect size would be 151×151 pixels (or bigger), but that is not that crucial. Just don’t use the biggest images you can find, because this can return you some problems.
Let me know how it goes, take care and have a lovely day,
Petra