Need help: filter pixelation and compression artefacts on upscale

[Chocobo]

Hey there,

I am doing a presentation of a painting in a realistic comic style. Unfortunatly I have to upscale the image x8 and that leaves me with ugly pixalation and compression artefacts.

Can someone suggest how I can treat the picture to hide those artefact, while taking the least invasive proceedour. I dont want to alter the original look too much. This is the problem I am dealing with: