Does anybody know or can do it? I have several pics I would like to upscale since I dont have the originals but it gets really bad with gigapixel bc of the artifacts.
I tried some AI programs but they just make it worse.

>>1533648 (OP)
These models are scary good at it:
https://openmodeldb.info/models/1x-JPG-80-100-alsa
You can run them with chaiNNer.

They manage to remove JPEG artifacts without smearing genuine details too much, if at all,
The 80% versions and below will also upscale 2x2 chroma downsampling.
The numbers are the range of quality the model covers, but I found it best to go down a notch when there's chroma downsampling.
So for say, 85% w/ 2x2 chroma (Xitter JPEG quality), 60-80 works best.