Can anyone throw some light on this, please?

Every so often I want to resize an image/object but when I try to grab a corner handle with what should be a double headed arrow pointer, I get a cross-hair and four-way arrow.

The image/object can be resized but I can resize it out of proportion which is not what is desired.

I'm wondering if I have used some obscure key shortcut to change this but I can't work out how I change it back.

Geoff