Image quality issues

Hi, I wonder if someone can help with what I’d expect is a straightforward query….

I’m a photographer and a blogging newbie. When I upload an image and specify ‘Full Size’ it appears on the page but with distinct ‘jaggies’. Clicking on it opens the image and it looks fine. Selecting ‘Medium’ size also works fine, but it’s a little too small on my page.

Is there a way I can post the image full-size without jaggies?

thanks in advance for any help
David