I think
keygrip wants the file size reduced, not the picture size. This is not 'resizing' as per photo editors, but compression. Converting the file type to a jpeg will do this, but only a few editors actually let you specify the resultant size. I have an old MicroGrafx Picture Publisher which has a wonderful slide bar control when saving jpegs. Lets you control the file size down to 1k !
Unfortunately I don't think this product is available anymore (and not for free as mine was). Are there any current editors providing a similar level of control ?