Panels can be positioned on a page using the blue guideline that appears next to existing panels when dragging and dropping the new panel. This guide indicates where on the page the panel will 'snap' to.
Lt organizes panels into columns. When you want to add to an existing column, drag the panel so that the blue line is only under the column you want. The size of any panel can be adjusted using the handles (thick yellow lines) of a selected panel. Images can be resized within their panel using the same yellow handles.
Where possible, resize images so that they fill up the entire panel by dragging the bottom handle down as far as it will go - this will make it easier to line up panels on a page. The whitespace panel is also handy for adjusting the layout of your page.