Looks like your changes are bein overwrited by the style.css file of your theme.
so basically it doesn’t matter if you set a image to the center if your css file says all images must be on the left thats what they are going to do.
a thing you can do is inspect your image with the elements inspector of your browser and get the aling class and see if they are been overwrited by some css files.
BTW you can try this.
/* == WordPress WYSIWYG Editor Styles == */
.entry-content img {
margin: 0 0 1.5em 0;
}
.alignleft, img.alignleft {
margin-right: 1.5em;
display: inline;
float: left;
}
.alignright, img.alignright {
margin-left: 1.5em;
display: inline;
float: right;
}
.aligncenter, img.aligncenter {
margin-right: auto;
margin-left: auto;
display: block;
clear: both;
}
.alignnone, img.alignnone {
/* not sure about this one */
}
.wp-caption {
margin-bottom: 1.5em;
text-align: center;
padding-top: 5px;
}
.wp-caption img {
border: 0 none;
padding: 0;
margin: 0;
}
.wp-caption p.wp-caption-text {
line-height: 1.5;
font-size: 10px;
margin: 0;
}
.wp-smiley {
margin: 0 !important;
max-height: 1em;
}
blockquote.left {
margin-right: 20px;
text-align: right;
margin-left: 0;
width: 33%;
float: left;
}
blockquote.right {
margin-left: 20px;
text-align: left;
margin-right: 0;
width: 33%;
float: right;
}
ALso check this
https://digwp.com/2010/05/default-wordpress-css-styles-hooks/
