The Image Preview dialog appears when you choose the Save command on Clipboard Viewer options.
If clipboard's content is a supported image format, it shows you an "Image Preview" window. Here you may select which format (of most popular) use to save the clipboard as Image. Click on proper tab to select target format as JPG, BMP or GIF.
Each image's format have their own proper parameters. Change these parameters to fit your needs, you may see a preview before save.
JPG: You may select the quality of the saved image moving the slider. The more quality, more space needed to save the image. Another option is save the image in gray scale format.
BMP: available option to change pixel format for Bitmap content, and save image as Monochrome bitmap.
GIF: You may choose the Color Reduction method, Gif format's compression method (LZW or RLE) and Dithering.
In all options you'll see the file's size (based on your settings), and the image size in pixels (Width / Height).
Next to preview box, you'll see a vertical slider. This allows you to zoom the preview image to see more details of the resulting image based on your specified parameters. Actual size is 1x.