For most of your galleries, where you have a set thumbnail dimension, your thumbnails will use the gallery images to fill the available space. This means that your images will be cropped to fit the thumbnail. If, however, you want to show the entire image in the thumbnail then you can do so by changing the Thumbnail Fill setting.
This will usually mean that your image won’t fill the entire thumbnail, leaving empty spaces on the sides. To improve the appearance of this, we’ve given you the option of filling this space with a color.
This will give you a gallery that looks something like this:
Using Thumbnail Fill
To implement this, go to the Advanced tab in your gallery settings.
Under Thumbnail Cropping, select Background Fill (No crop). You can then select the fill color from the drop down color selector. Once you have chosen a color, update the gallery to reflect your changes.
As you will note, this will only work if your are using the GD image library. To find out how to enable GD on your system, read this article.