Image+ is an advanced image custom template variable type for MODx Revolution. The dimensions for the uploaded image can (optionally) be configured to constrain a minimal width and/or height. The image crop could be forced to remain at a pre-set ratio. A graphical tool could be used to crop the image to the required dimensions/proportions.
Install through package manager and select 'Image+' as a template variable input type and output type if you create a new template variable. Ensure you also have a supported cropping engine (pThumb, phpThumbOf, phpThumbsUp) installed.