Within your concrete5 dashboard, you are able to configure your thumbnails. Meaning, automatically generate thumbnails based on your width/height. But each theme has its own breakpoints for tablet/desktop/widescreen… So it makes sense to display these thumbnails for different browser widths, right?
That’s where responsive image maps kick in. In this tab you can add maps and for each map you enter a “Handle” (which is a handle you created upon configuring a “thumbnail”) and the “Minimum Browser Width” – in pixels.
You can visit the “Supporting Responsive Images in your Concrete5 Theme” documentation page for more information about how to create your thumbnail types and how the things you enter in the form get translated to the page_theme.php class file.