Looking at the source code of the module containing the images on this page - https://triple-group.nl/nl/wat-doen-we/finance - they are not all set at 120px wide. I would recommend changing the way you handle this, by inserting the images without dimensions (or with only a width value of 120) and then using css to set the image width for all images in the module. For example, adding the following to your template stylesheet should work:

#sidebar .custom img {
    width: 120px;
}

Assuming you are referring to the image in the article "natuurvriendelijke oever", it appears that the image is being displayed in the front-end with the full dimensions. Perhaps this is being done by another plugin you have installed on the site?

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I am getting the same result on your site whether the editor is set to JCE or Tinymce. Although the image dimensions are set to 320 x 225 in the editor, and saved with these values to the database, the image dimensions are reset in the HTML code to the images full width and height.

As the editor is not active when the content is rendered in the front-end, I can only suspect that there is something else on your site that is altering the image dimensions as the code is rendered to the page.

I cannot find a specific plugin that might be doing this, so I think it could be something in your template.