Hi, Gernot 

Firstly, I would like to sincerely apologize for the delayed response as we have been experiencing an unusually high number of tickets. I am sorry that it has taken longer than usual to respond to your concern and your patience is highly appreciated.

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You are correct, this is one of the current "default behaviour" issues that our devs noted on our "Editor types" Documentation here, regarding the image column being used with attachment editor type.

- If the Column type is set to Image, and the editor type as attachment, 

the uploader will automatically insert a ‘thumb’ size in the table cell (the resolution is defined in WP config), and link it to a full-size image. 

If you have a WP Lightbox 2 plugin installed, each image will automatically open in a lightbox.

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If we don't use the "attachment" as the Editor type, for example, if we use one-line edit, or multi-line edit;

then we can manually implement in the cell, if we wish a thumbnail, or the path to the full image without the thumbnail,

- As we stated in this documentation about our Image columns;

the specific rendering rules for image cells are

: If the content of the cells contains a combination of “two sticks” – || – everything to the left of this combination is used as a link to the thumbnail, and everything to the right as a link to the full image size:

http://domain.com/thumb.png||http://domain.com/fullsize.png

If the “||” combination is not found in the content, the entire content will be used as a link to a full-size image.

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So, unfortunately, this is the current limitation - you can try to change the default "thumb image upload" resolution is defined in WP config.

- If you wish, you can read more details on how uploading image sizes works in WordPress here.

I hope this helps.

And if you have any suggestions or feedback on how we could improve the plugin > 

Please feel free to search on our suggestions page, 

 to see if someone may be already suggested this feature. If you can't see it, feel free to add your suggestion there,  and as more people vote, the feature will move higher on the priority list.

You can certainly follow our changeLog page if you'd like ( it is also available in the plugin dashboard), where we state any changes/new features/bug fixes during updates;

and our newsletter, so you're informed about new features, bug fixes, freebies, etc.

Let me know if you have any additional questions. Thank you