Custom image style is not honored on new content

Question

On drupal 7 I created two new image styles. Now when I save new contents, I expect the new images to be saved in the new image styles as well but this does not happen. Images are only saved in the ordinary thumbnail, medium, large and square-thumbnail sizes.

How can I make the new image styles be honored?

Edit: So Mołot and Bala made it clear that a certain image style has to be called to generate an image of that style. Yet, I would like to call for a smaller image in the code, and then load a larger version for wider screens.

Is this possible? Note: I am using ZEN as the theme.

Edit 2:

What I want to do is to load an initial small image like:

<img class="resp-image" src="very-small.jpg" />

Then, according to the screen size, I will replace the image with a larger version. This version of the image is provided by an image style:

if($(window).width() > 768) { 
    $('.resp-image').attr('src', 'large.jpg');
});

The problem is, since I do not refer to large.jpg in the content type's display settings, it is not rendered to the given style and thus does not exist yet.

I was going to put this up as a new question but I found out that it is already discussed here and here.

Mołot's answer best described the situation referred to in the inital question. So I accepted it.

Answer 1

Images are only rendered to a given style when something actually tries to use that style. If you want to force generation, you need to use image_style_url to obtain URL for the image, and then download it via regular HTTP request. It becomes more complicated if you don't want to download (aka show) it. In that case, use image_style_deliver.

Answer 2

Go to manage display and set the style you want under the format, see the below screenshot and choose the image style you created.