Just like at my forum.   Looks ok in preview, but once posted, I don’t get the new paragraph. The code:

 <p>Testing how it works here:</p>
<p><img alt=”” src=”https://simple-press.com/wp-content/themes/SP2015/images/sidebar/custom-plugin.png” /> <img alt=”” src=”https://simple-press.com/wp-content/themes/SP2015/images/sidebar/custom-theme.png” /></p>
<p>Now I should have a new paragraph.</p>

Why isn’t it a new paragraph?

what alignment are you applying to the images?  looks like left (floated), which is what we use here as default…  so things stack up on the left just like you see…

you can change the default thumbnail styling on forum – options – content settings…

and you can also hit the html button in the editor and format it how you want…