First off, love your app - it's fantastic to get an ever-changing desktop wallpaper. And to have such control over what images/scenes are selected.
But there's a feature I'd love to see added - something that's an option in an older, less flexible Flickr wallpaper changer. See
An option just like the shown in the screenshot would be fantastic. Choose a color to overlay onto the downloaded wallpaper and choose the opacity/transparency of that color. So you can lighten, darken or recolor any image.
Many images from Flickr are just too bright and desktop icons get lost. Being able to fade the wallpaper would be an awesome feature!