I noticed that course images are converted to PNG after uploading. I don't want to sound picky, but I think JPG would be a much better choice, because (at least here) course images are often photos that result in too big files.
For example I created an image with the suggested dimensions, exported it with 75% JPG compression and run it through ImageOptim (a very nice collection of open source command line tools to squeeze the very last byte from images without losing quality). This results in a small 30kb file that OpenOLAT converts to a 340kb PNG.
Of course there a images that work best as a PNG file, so maybe it would be an idea to just keep the uploaded file format (use PNG if the uploaded file is a PNG and keep JPG if the uploaded file is a JPG)?