Here is an image of my laptop keyboard. The image above is inserted as is, without any CSS “classes”. Some themes automatically format images when no IMG class is assigned, while most do not. The image is 240 pixels wide, which should fit most themes–even those with narrow main body columns.
Now here’s something that makes my mouth water everytime. This image is 500 pixels in width, and is a great gauge on how wide an image that a theme can handle. Some themes automatically limit the width (or even the height) of images via CSS, but most will not. So you better be sure your images fit, or they will run off into the sidebar and/or break your layout.