I'm unsure why this is the case, but I've coded in an image into my Debut theme, and after a while of being there on the website the outside edges of the image become jagged and pixelated, almost like Shopify has increased the sharpness of the image.
Here is before and after:
This happens after a little while of it being on my website, the original image definitely hasn't got jagged edges.
Image size is 20% of the page width.
Also when you zoom in on the second image when its on my website the edges seem to smooth out.