how to delete images that were uploaded but low resolution and can not be used

At some point the required resolution has changed for images. I have a bunch of images in my files that are low resolution. I would like to delete them from my media but there is not check box to select them. How do I delete them?

I find on some machines I see a checkbox and on others I do not. How are you accessing the site? 

@Nancy7  if you hover over the image - the checkbox magically appears.

Then select the ones you want to delete, then scroll to the bottom and press the delete selected button

I just went in to try your solution and all the grayed-out images are magically gone! Some of them have been there for months.  Must be the Zazzle elves read my question and fixed it for me!

Thanks for trying to help.