It's a bit of a pain to find.

 

Example:

I click on the Image Gallery button from the editor (not the insert image because it's an existing one).

I choose a photo which drops it into the editor (but it's the wrong size because I want a thumbnail)

So I click on the image which takes me to the image properties

Now I have to click file browse to get to the screen with the image editor and thumbnail icon on

Now I have to find and select my image again because it isn't selected by default.

 

 

I could use the insert image icon instead of the image gallery one to get this done in less clicks, but I would naturally have thought to find this icon in the image gallery as well.