upload imges onto webpage

I have a question about uploading images (jpeg or png)onto the webpage.
So I am trying to convert doc or pdf file into jpeg or png, and upload them onto my webpage. But after I upload the images onto the webpage, the words in the images always look fuzzy and not so clear. Maybe because that I also enlarge the images before uploading.
But do you know why this may happened? Or any tools I can use to enlarge the images without lose the quality?

Thanks a lot!