Size of Images in Posts

Here is my problem:

Whenever I add media to a post, the size of the is hard-coded in the post. For example, let’s say I have defined the medium size to 640 x 480. The code for the image in my post will look like this:

<img class=”alignnone wp-image-666 size-medium” src=”https://https://mysite.com/wp-content/uploads/posts/imagetest-640×320.png&#8221; alt=”imagetest” width=”640″ height=”320″ />

But since I changed my theme, it seems appropriate to change the settings for image sizes. However the posts remain with the same hard-coded sizes.

For now I have manually updated all posts. But what suggestions do you have to prevent this in the future?

Edit: Okay, I figured out that using the code like this provides a rough solution for my problem:

<img class=”alignnone wp-image-666 size-medium” src=”https://https://mysite.com/wp-content/uploads/posts/imagetest.png&#8221; alt=”imagetest” />

The only problem is that it loads the full image intead of a preview, but it is displayed properly.