Why can’t I resize the images?

The size is locked to prevent quality loss when images are scaled to a smaller size and than to a bigger size. Perhaps in the future the scaling posibillities will be improved.

What will happen to my uploaded images?

At the moment everything happens on your own computer ( client side ) therefore nothing is uploaded to the server, perhaps this will change in the future, with an optional share function.

Why is the app sometimes seems less responsive.

The performance of the app has a lot of dependencies, the computer, the number of images in the wallpaper and the resolution of the chosen wallpaper. It could help to scale images down a bit. For most wallpaper resolution a 3Mpixel photograph (2048 x 1536) will be overkill, especially if more photographs are uploaded. However if only one image is uploaded to a large resolution, for example the Macbook Pro Retina (2880 x 1800) the wallpaper will fail, because the image will be too small, so for large resolutions 4-5 Mpixel will be sufficient