It probably didn't allow you to drag because you ran Slimtex as an administrator.
Then your explorer window is running in the user token and then you are not allowed to drag from one token to another because of security.
If you think about it it makes sense.
As for your other problem: Start by sorting on resolution and only select the files in the window that have a (too) high resolution.
Then select to resample them by 1/2
I'd rather have a bottle in front of me than a frontal lobotomy