How to not lose image quality?

When in the ghost editor, before uploading articles, my images are fine, their quality is fine
But when I upload the articles, the images lose quality, sometimes considerably, and I do not want that.
How can I solve this?
I’ve heard that there is some image optimization thing, but I never get disabling it right. How can I do it?
I’d like to keep them having the same width they have in the articles, but maintaining the quality. Is this possible? and if yes, how?

Which theme? It’s possible your theme is loading the wrong image size for the spot the image is going into.