crixul

Add this line to your htacces if you didn’t already.
RewriteEngine On
RewriteCond %{SERVER_PORT} 80
RewriteRule ^(.*)$ https://dia-archief.nl/$1 [R=301,L]
Modify on your WP Dashboard Settings>General https://dia-archief.nl instead of http.
Then delete or hide the header image, save publish, then put the image again, will have the new https.
If you are running some other scripts you need to disable or delete and re-enable, just for the modules to read the new settings from general using https.
Hope it works for you.

You can’t force resize, instead crop all your slider pictures at the same size something like 1/3 width/height using picresize.com or something, and your slider will look more professional in 3 minutes.