did you insert the image with an alignment, such ass ‘left’ or ‘right’?
does your theme’s style.css have the styles for these alignments, such as:
.alignright, img.alignright { float:right; margin-left:10px; }
.alignleft, img.alignleft { float:left; margin-right: 10px; }
.aligncenter, img.aligncenter { display:block; margin-left: auto; margin-right: auto; }
or very similar?
there could be some other style in your theme, that interfere with alignment.
if that does not get you going, you could post a link to your site with the alignment issue showing.