Can't get post thumbnails to work

I’ve been struggling for days with this problem. I’m an old Blogger, new to WordPress and I’m not ashamed to say I’m lost. The only way I can get post thumbnails to show on homepage, category pages and search pages is to go back to Blogger, copy the image to my desktop, upload it to media library and set the featured image.

With over 360 posts I will be working for a long time if that is my only solution. I’ve searched Google and WordPress and can’t find any other solution, and I don’t want to change my theme.

This is my website:
civilwarwomenblog.com

Email: [email protected]

Any help will be greatly appreciated!

Thanks,
Maggie MacLean