hi
There may be a way more automated than what I am suggesting here but this will work.
in your stylesheet style.css in the theme folder, add one line
.clearleft { clear: left; }
Now, when you want a paragraph to appear under a photo instead of next to it, after you have entered the paragraph in the Visual Editor, click on the editor's HTML tab.
Where you see this:
So, taking a page from brave and crafty women, I decided to take a few of my battered photo frames and spray paint them all the same color so as to add a little accent to the wall. I chose white because I’m timid that way, and because it seemed to make sense. The trim and ceiling are white after all. By themselves, the photos look okay, I think. But just okay. Not cute.
change it to this
<p class="clearleft">So, taking a page from brave and crafty women, I decided to take a few of my battered photo frames and spray paint them all the same color so as to add a little accent to the wall. I chose white because I’m timid that way, and because it seemed to make sense. The trim and ceiling are white after all. By themselves, the photos look okay, I think. But just okay. Not cute.</p>
If you want to install a plugin that will make the process easier, it is this one:
http://wordpress.org/extend/plugins/tinymce-advanced/
It adds some buttons to the editor. One of them allows you to directly assign CSS to a section of text. You could highlight the paragraph and assign clear left to that section of text.
Either approach will work. Perhaps someone else can suggest a simpler way.