DrawTool | a drawTool by openGL

1) Image dimensions are hard-coded now: how do I feed image dimensions from python to js?

2) The image is transferred to the client twice: first as an argument to image(), then as source for the button plot. How to access the image "source" from the DrawTool?

The answer to these is the same, add more properties (on both the Python and the JS sides) for the data you want to store on the DrawTool. E.g. another Instance for another ColumnDataSource that holds the image data, and integer properties for the width and height.

3) If (all this code being in one jupyter cell) I run it the second time it refuses to plot anything with a javascript error in console Model 'DrawTool' does not exist. Running it the third time, fourth time and further on works fine. What exactly is bokeh trying to tell me in this error message?

This message is stating that BokehJS does not know anything about any DrawTool, and the reason for this is that, due to the way things work in the notebook, custom extensions only get registered when you call output_notebook. So you will have to call output_notebook again after you define the custom extension. I don't like this state of affairs but there is nothing we can do about it.

Or how is it possible to specify more than one css class to such a custom action icon?

It's not possible, currently. The built-in ButtonToolButtonView code does this:

Offical definition from dojo

startup(): Probably the second-most important method in the Dijit lifecycle is the startup method. This method is designed to handle processing after any DOM fragments have been actually added to the document; it is not fired until after any potential child widgets have been created and started as well. This is especially useful for composite widgets and layout widgets.

When instantiating a widget programmatically, always call the widget's startup() method after placing it in the document. It's a common error to create widgets programmatically and then forget to call startup, leaving you scratching your head as to why your widget isn't showing up properly.

So as Kirill mentioned, you need to call the startup method.