Bill asked me today how it’s possible to use a certain post thumbnail size in SNIP. Unfortunately that is not possible at the moment directly within SNIP. However there is a workaround that extends its functionality. And here is how it works:

I wrote a little helper plugin that you can download here (Download ZIP directly).

1. Log-In to your WordPress Dashboard.
2. Go to Plugins -> Add New.
3. Click “Upload Plugin” from the top (right beside the headline).
4. Select the downloaded ZIP file.
5. After the plugin has been uploaded, you can activate it.

Then, edit one of your Global Snippets:

1. Open up a Global Snippet.
2. Add an “image” property to your Structured Data Snippet.
3. Select “Post: Thumbnail URL (size)” from the “Field Type” dropdpwn.
4. Enter the name of the thumbnail size into the “Field value” textarea.

Per default you can use full, post-thumbnail, medium, medium_large, large and thumbnail.

Other plugins and/or themes can also add new images sizes. Just ask their developers or install a plugin that shows the images sizes, like Display Image Sizes in Media Library.