I add a photo to a post and it slid off the page.

Can you replicate the problem using the default Twenty Thirteen theme with all plugins deactivated?

You mean the post-thumbnail?

I’m pretty certain this is by design, the images in Twenty Fourteen post content that are right or left aligned are meant to “pull” out of the main column a bit.

I found the problem, there is a bad css rule.

.full-width .site-content blockquote.alignleft, .full-width .site-content img.size-full.alignleft, .full-width .site-content img.size-large.alignleft, .full-width .site-content img.size-medium.alignleft, .full-width .site-content .wp-caption.alignleft {
margin-left: -168px;
}
If i deactivate this margen-left rule then the photo slides back in to place and stays in the confines of the div tag.

Yes, that’s the CSS rule I mentioned that does the “pull” style to make the image left or right aligned. It’s intentional.

I add a child them and removed that css rule. I think is looks better confind to the div/ mini page.
.entry-content img {
margin-left: 0px !important;
}