In principle, the plugin should work with all themes, assuming they insert the featured image using WordPress’ built-in function “get_the_post_thumbnail” (which they usually do). Now, when using that function, it is possible that external featured images do not use the appropriate size in different pages of your WordPress site (which I understand is the incompatibility you’re talking about).
WordPress creates several thumbnails -different sizes- for each image in the Media Library, and themes request one version or the other depending on what they need at each point (for example, they use a smaller version in the index page, the full-sized image in the post, and so on)
In order to modify the featured image and use an external URL, we have to hook to WordPress’ “post_thumbnail_html” filter. We’re currently trying to improve the sizing issue with the information we have at this point, but I’m afraid we cannot guarantee that the result will work properly everywhere.
As a “simple” solution to this problem, I suggest tweaking the CSS of your theme so that images inside a div with the class “nelioefi” have the appropriate dimensions. I understand this CSS-tweaking requirement is something we should include in the plugin description, so that you guys can be aware of the limitations we’re currently facing.
I’m sorry the description was confusing and the result was not as good as you expected. Please, let me know if I can help you with anything else!