When using the Gallery theme, each asset thumbnail is rendered to the DOM as follows: A thumbnail container element is drawn with the dimensions specified in the “Thumbnail dimensions” section of the Gallery theme options. The thumbnail strategy defines the rest:
When in Scale mode, an image element whose source is the asset’s thumbnail is drawn inside the bounding box. The image is scaled to fit within side the bounding box while preserving the original image’s aspect ratio. Once the image is scaled to fit within the specified bounding box, the bounding box will collapse around the scaled image (unless the “Center-align thumbnail (letterbox)” option is checked, in which case, the bounding box will remain its specified size, and the areas of the bounding box that do not overlap the scaled image will be shaded in with a dark gray color set to the opacity specified by the “Pillarbox and letterbox opacity” setting in the Wiredrive Player Settings admin page).
When in Crop mode, the thumbnail image is used as the horizontally-and-vertically-centered background image of the thumbnail bounding box. This means that the dimensions you specify for the thumbnails now act as the dimensions for a horizontally and vertically centered cropping mask over the original thumbnail.