There’s no way to crop a JPEG image without recompressing it, at least that I’m aware of.
And there’s no way to warn about the destructive save- though it’s something I’ve been considering adding in a near term release. Basically, any images that aren’t native to Acorn would open up as untitled windows, and you’d have to do explicit save over the source image to write it back out again to the same location. Is that what you’re after?