Thanks! There's plenty of software that supports these features (and does not require you to touch any--gasp--java). I am just sad that these basic things are not readily available in GIMP, which could otherwise attempt to be a sort of generic tool for image processing. They seem to be focused too much in color photography, while the tool is not directly useful to people working with images that are not of this kind (e.g., astronomical images, microscopy, satellite, radar, and mathematical image processing).