Arthur, welcome to TalkGraphics, your Blender experience (your first Post) is most welcome.
In this case, you are all chasing rainbows, putting stuff back into images that was never there.
Surely, the noddy solution is to ensure the image is displayed at its natural dimensions before applying the mould.
I have just scaled the result down by a factor of four and while you can still see jaggies you have to zoom to 400% to do so.
To go a step further, use @handrawn's suggestion to apply a Live Effect at 300dpi: I used Gaussian Blur (smart), Radius 8, Threshold 255.
All Xara.
Acorn
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