This way you don't need to download any third party software apps or programs. This is all in your computer right now. I'm on a windows 10 PC.
Put your pic on your desktop screen.
Right click on your picture, click on 'send to' then Select 'mail recipient' in the pop up box.
A picture size box pops up. Select the 'Smaller 640 x 480'. Click 'Attach'.
A message screen pops up like you are going to send the pic to someone, but you aren't going to send an email.
Highlight (click on) your resized picture, right click and select copy.
I have a folder I put my resized pics in. I left click in the folder and select 'paste' to drop the resized pic in the folder.
Now the pic is ready to be used in your Cast Boolits posting.
This whole process takes about 30 seconds. Easy-Peasy !!