On the layout page you specify what your HTML pages should look like. Most of the options on this page are straight forward, and do not need further description here. A short intro to each theme is shown when you select the theme to give some idea what it should look like.
At the bottom of this page it is possible to specify several resolution for images. This way people reading your pages are not forced to download images in large resolutions, say 1600x1200, just to see it on a monitor not offering more than say 800x600.
At any time your readers may change resolution among those you specified. That way they may browse through your images in low resolution (and thus fast download time), and when they see an image of special interest, they may change to high resolution.