Image size (if srcset is removed or not)

I choose an image (size: 1920 x 1280 px) from media-library. I hit update and look at the front, the image appears. Ok. Now I go back to edit and choose the thumbnail medium size (300 x 200 px). It does not show up in the backend and after update the document it does not show up at front. The source tells me

.../image-300x200.jpg" alt="image" width="1920" height="1280">

Did I something wrong?

Hello @Mario,

I hope I’m understanding this correctly.
Basically, when you change the thumbnail size, you are not seeing those changes applied to the image?

This is because we’ve chosen to use the thumbnail size property as a way of specifying which file size you want to force on a user.
It doesn’t modify the width and height properties (which are applied through the Size tab).
The width and height attributes you see on the img element have no effect on the image itself (unless you remove the css height and width properties), they are solely used to indicate the ratio of the image so that CLS doesn’t occur.
This way, an image can keep its 100% width property and not affect the layout when loading.

Hope this clears things up a little.

Thank you for your answer, Louis. It is still confusing to a new user like me. Below you’ll find my points:

The picture shows the behavior of a WP-image (everyone knows). I choose my custom image size and the dimensions are filled automatic in. No more to do here. The image, which the editor shows, is the one, witch dimensions I choose (480 x 320 px).

Here I also choose my custom image size in the third tab and activate remove SRCSET. But then I need to fill the size by hand in the second tab. The picture shown in the editor is not the one I choose, it is the fullsized version (1920 x 1280 px). Where to enter the caption?

In summary (for me), the first attempt is more intuitive, more streamlined, easier to use.