mogrify leaves me with FILENAME.JPG~ ... does not overwrite

[flipjarg]

I am hoping someone can at least give me some direction on this. I am not very familiar with using ImageMagick on Windows, so I may be missing something obvious. Right now I have a right click menu that runs this command to resize any image(s) that is selected.

"C:\Program Files (x86)\ImageMagick-6.8.9-5-Q16\mogrify.exe" -resize 640 -quality 100 "%1"

This works just fine on my computer. However, this morning, I installed ImageMagick on a friends computer and setup the menu for him. When I run it on his computer, I get the following result...

big_photo.JPG
big_photo.JPG~

Instead of resizing the file, it just creates the file big_photo.JPG~. I renamed the file to "big_photo_resized.JPG" and it has been resized and opens just fine. I just don't know why ImageMagick isn't overwriting "big_photo.JPG" with the resized image.

More info:
Operating System: Windows 7
ImageMagick Version: ImageMagick-6.8.9-5-Q16-x64-dll.exe

[flipjarg]

...okay, I was partially wrong about this. Apparently the original file is being resized but it the file "filename.JPG~" gets left behind. If I resize "filename.JPG" again, it creates another file name "filename.JPG~~" and so on. For some reason this temp file (I assume it is a temp file) is not being deleted. Any ideas?

[snibgo]

Permissions? Maybe the logged-on user doesn't have delete privilege on the file or from the directory?

[flipjarg]

This is being done in a folder on the user's desktop. I can't imagine it would be a permission issue.