You can alter the size and quality of thumbnail previews that Retrieva generates for your media using the Preview Width and Preview Height text boxes and the Preview Quality slider.

The image on the right of the window serves to show a preview of the Preview Quality setting, and this image can be changed between 3 different images using the Test Image slider so you can see the effects of the image compression setting on different kinds of images.

Using the Zoom slider you can zoom in on the preview image to better observe image compression artifacts that become more apparent the lower you set the Preview Quality.