Portfolio provides thumbnail, which is in indexed PNG — which results in only 256 colors possible. You can clearly see mosaic on zoomed in gradients.
It's possible to extract original quality image from this site (also manually, but it's harder) with this script (see first post for download links). You can do it manually using this link https://portfolio.commishes.com/image/NUMERIC_IMAGE_ID/original/ — like this
Keep in mind, that original can be worse — for exampled image thumb is 1920x1847 PNG, but original image is 1717x1652 JPEG — smaller and jpeg. Thumbs are not only in unsuitable format, but also upscaled — very easily noticeable here.
Conclusion: use script to obtain images from this site when possible.