Select an image by clicking to the right of its name. If the image is able to be thumbnailed, or if a thumbnail does not already exist for it, the Create Thumbnail button will appear

Click the Create Thumbnail button. The Thumbnail Dialog will open.










The Size value (in pixels) that you would like the largest side of the image to be resized to. For example, if the image is a 800px x 600px image, using a resize value of 600px will resize the image to 600px x 450px

The Quality select list is the jpeg quality value (in percent) of the final image. This value can affect file size.

The Type select list allows you to select the thumbnailing method.

Proportional generates a thumbnail of proportional dimensions, ie: as mentioned above a 150px x 113px thumbnail will be genertated from an 800px x 600px image.

Square generates a square thumbnail of the chosen dimensions, ie: a 150px x 150px image. This image will be generated by resizing and cropping the image.
Set the desired paramters for the thumbnail.

Click the 'Ok' button to submit the request. Click 'Cancel' to close the dialog and cancel the action.
When a thumbnail is created it is created inside a 'thumbnails' folder (this name can be set in the Image Manager Extended configuration) in the same folder as the source image. If this folder does not exist it is also created. 
Deleting a Thumbnail

If an image has an associated thumbnail, selecting it will display the Delete Thumbnail button

To delete the thumbnail, click this button.

A confirmation dialog will be displayed

Click 'Yes' to confirm deletion, 'No' to cancel.
When a thumbnail is deleted, if the thumbnail folder it was in is left empty, the folder is also deleted.