Image size incorrect [Important!]

I used other thumbnails, now the problem has changed: One thumbnail is 210x135pixels and the other is 217x135pixels big. My theme specifies a size of 220x135pixels and the images have the different sizes on my computer but this isn’t supposed to matter.

I guess you have unchecked “Crop thumbnail to exact dimensions (normally thumbnails are proportional)” in YOUR WORDPRESS->settings->Media which you may refer to Marg’s Explanation.

If checked -> crop the size exactly as stated
If unchecked -> crop the size proportionally based on the longest side.

And after you update the settings, I suggest you to use plugin like Force Regenerate Thumbnails

You may refer to WordPress Image Size and Featured ImagesWordpress Image Size for Custom Image and Featured Images for more information which is being written.