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create a figure and "Save figure as" with 300 dpi
The resolution is obviously wrong and some fractal pixels are displayed in areas were colors are changing.
Some customers need a better result here.
(our competitor 3Dvia Composer creates better results)
Similar idea here: Add png as a save as file type
pngs are better suited for graphics than jpgs are. FYI - TIFFs are supported so you could always use those and your images will look better.