Hold down the Shift key as you drag handles.
It's a simple trick when you want to get on with your project quickly, without endlessly going to Inspector and switching from Graphic to Metric, to Wrap, etc.
Constrain proportions function is in Metrics inspector, not Graphics. Put a tick in the checkbox next to 'Constrain Proportions'. It stops the graphic image distorting when it is resized – if you don't, the image will be squashed, when you pull horizontally, or stretched, if you pull vertically.
Holding down the Shift key constrain proportions temporarily, only for the time you need it.
This trick, of course, is for pulling on the object handles in the middle of the sides of the object, not on the corners.