You can optionally toggle on a preview image area that will show you in real size what the currently selected image will look like. This will show you whether your image is going to look to pixilated if it was to small to fit in a cell.
You can toggle this on from the Window menu.
Note: If you see a red square in the upper left corner of the preview window this means that your image had to be stretched to fit into the assigned cell. When an image is stretched it may not appear as crisp and clean when printed.