There is a line in the style.css file about 3/4 of the way down in the file...
div.post img { margin: 15px 0; border: none!important; }
I'm almost 100% positive that this is causing the issue that you are having. If you go into the Editor which you can find under the Appearance tab in the Dashboard on the right hand side of the screen you should see a list of files. In that list you'll see style.css listed somewhere.
Click on the name of the file so that it opens in the textarea in the middle of the screen. Copy all of the content within the file and paste it into a separate document on your computer. This is in case something gets messed up while you make the changes, you can use it as a backup.
Press Ctrl+F on your keyboard assuming you're using a Windows computer and enter in the line of code I pasted towards the top of this response. It should take you straight to the code, once there remove the !important text from the line of code and then Update the file. This should fix the issue you're having, if it doesn't let me know.