Can't Center Image After WordPress Upgrade

At some point in the last month or two, images that were centered on my blog (which is my default) no longer show up as centered – unless they have a caption. I’ve looked into the CSS and tried some suggestions I had found (adding a float: center, changing margin-left and margin-right to auto, even adding a text-align: center to the class), but nothing seems to be working.

The class in question seems to be imgcentered:

img.centered, .aligncenter {
display: block;
margin-left: auto;
margin-right: auto;

And here’s a page on my site that has an image with no captions: https://www.50by25.com/2014/07/reviewgiveaway-adora-calcium-supplements.html

For comparison, here’s the class for captions:

.wp-caption {
  padding: 0;
  text-align: center;

And here’s a page on my site that has images with captions: https://www.50by25.com/2014/07/weekend-recap-fitness-on-the-rocks.html

How do I get regular images with captions to center?