From FCKeditor Docs

You can set configure thumbnails by using the following settings in the configuration file:

Thumbnails.Url = BaseUrl + "_thumbs/";
Thumbnails.Dir = "";
Thumbnails.Enabled = true;
Thumbnails.MaxWidth = 100;
Thumbnails.MaxHeight = 100;
Thumbnails.Quality = 80;

Url and Dir

The Url and Dir properties defined the folder used to hold all thumbnail images, and the URL to access them. It works just like the BaseUrl and BaseDir settings.

Enabled

By setting Enabled to "false", CKFinder will have no support for thumbnails.

MaxWidth and MaxHeight

Sets the maximum size of the thumbnails displayed in CKFinder. These values are expressed in pixels.

Quality

Sets the quality level used to produce thumbnails in a range from 1 to 100. The smaller the quality the smaller the size of the thumbnail files. Notice that an acceptable value should not be less than 45. For quality thumbnails, 80 is more than sufficient.