Most Ghost themes assume that they’re receiving an image big enough for any spot they’d like to use it in. So when you uploaded a very small (low resolution) image, it looks bad in that spot.
Something like this (in the page’s code injection spot, or site-wide) will avoid the image growth:
<style>
.gh-article-image img {
object-fit: contain;
width: unset;
margin-left: auto; /* these two center the image - remove if desired*/
margin-right: auto;
}
/* align captions to center, because they look weird otherwise:*/
figcaption {
text-align: center;
}
</style>