<f:spaceless>
    <v:variable.set name="backgroundImagesURLs" value="{v:iterator.filter(subject: {})}"></v:variable.set>
    <f:for each="{backgroundImages}" as="backgroundImage" iteration="i">
        <v:variable.set name="url" value="{f:uri.image(src: backgroundImage.uid, treatIdAsReference: 1, absolute: 1, maxWidth:1200, fileExtension:'webp')}" />    
        <v:variable.set name="backgroundImagesURLs">{backgroundImagesURLs -> v:iterator.push(add: url, key: i.index)}</v:variable.set>
    </f:for>

    {backgroundImagesURLs -> v:format.json.encode()}
</f:spaceless>