Quote:If you want a comparison a painting would be an adquate one. The scan is unfortunatly 8 times to little resolution and was exported as jpg. It is simply not hires enough and there is jpg compression. I want a look that is more like the real item.
Sure I can do that. Using your image as an example, an image search brings up plenty of equivalent 'paintings' in small sizes.
Small size as downloaded (480x270) and zoomed in to 800% which of course shows the individual pixels. https://i.imgur.com/j4zu89K.png
If that is what you want scaled x8 (3840x2160) then using Interpolation None will keep the pixel shapes. https://i.imgur.com/my9pnfA.png but you were complaining about pixelation.
A more usual x8 upscale using LoHalo which interpolates the pixels is as expected a bit fuzzy. https://i.imgur.com/a1aXCQd.png but no pixelation.
... and this as mentioned the gmic upscale, very slightly sharper, x8 is a big jump. https://i.imgur.com/bNi5Fhr.png
Since you say it comes from a scan, nothing you can do about original quality. You would be better off getting a better scan and not using jpeg output. Most scanners will output a tiff. If that is not possible try improving the scan before upscaling, The gmic plugin again has filters JPEG smooth and Unquantize to try and repair.
Generally, a sad fact, poor image in = poor image out (trying to be polite here
)edit: This a screenshot of the gmic unquantize filter, removes artifacts but it is akin to smoothing. https://i.imgur.com/dcRRQNf.png