The images I upload always have a max height or width of 550px. Under options I can choose Image quality and Image size, but I don't need them. Doesn't it make the display of images slower because they get 'resized' and 'compressed' anyway?
It would be nice to have an option of just the creation of thumbs and just use the uploaded images as they are.

If the size in the options is larger than your uploaded image and "upscale" is not allowed they are not processed except for the thumbs.

The image processing happens only once per image, so I doubt that it makes much difference in performance. There has been discussion of directly loading images that don't need to be resized rather than placing them in the cache. That may happen, who knows. But it does cause some complications with certain configurations that have the albums folder outside the scope of the browsers.

OK, I will set the size larger than 550px.
Another reason why I asked this is that I save each image to it's maximum quality with minimum size (using the preview window in the Gimp or Save For Web in Photoshop). When each image would be 'rescaled' and 'compressed' again by Zenphoto all my work would have been for nothing .

I absolutly understand that reason...:-)