Größe in TImage

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Ahoj

Creating a project in which the user can draw a picture to RichEdit.
The position of "Row" and "Count" is automatically created to the page Paintbox.
At this Paintbox user will draw. Once you finish drawing and clicks the "Enter", so I need to crop the image automatically and only the coordinates, where an image and discard the rest.
It is unnecessary to file a 100 x 100 pixel screen takes up the size as 800 x 600 pixels.

Trying to get coordinates Mouse_Min_Top, Mouse_Max_Top, Mouse_Min_Left, Mouse_Max_left.
Once I know these coordinates, the problem is not crop the image.

Please show a little code.
Thank you very much