You can’t avoid compression in Wix any image you’ll upload will be optimized but images shouldn’t lose their quality much. If you need a better quality image try using PNG instead of JPG files.
PNGs are better at quality and Wix will compress them different then JPGs, this is what’s recommended by official sources.
Also you can try to upload JPGs in higher resolutions e.g. if your image is 1000x1000px try to scale it to 2000x2000 and upload again.