Kenn die Ext nicht, aber mit Fluid kannst du sowas doch ganz easy machen.
Zum Beispiel:
<picture>
<source srcset="{f:uri.image(src:'{file.uid}', treatIdAsReference:'1', cropVariant: 'default', maxHeight: '{data.imageheight}', width: '{dimensions.width}', maxWidth: '1920')}" media="(min-width: 1601px)">
<source srcset="{f:uri.image(src:'{file.uid}', treatIdAsReference:'1', cropVariant: 'default', maxHeight: '{data.imageheight}', width: '{dimensions.width}', maxWidth: '1600')}" media="(min-width: 1334px)">
<source srcset="{f:uri.image(src:'{file.uid}', treatIdAsReference:'1', cropVariant: 'default', maxHeight: '{data.imageheight}', width: '{dimensions.width}', maxWidth: '1333')}" media="(min-width: 1001px)">
<source srcset="{f:uri.image(src:'{file.uid}', treatIdAsReference:'1', cropVariant: 'default', maxHeight: '{data.imageheight}', width: '{dimensions.width}', maxWidth: '1000')}" media="(min-width: 801px)">
<source srcset="{f:uri.image(src:'{file.uid}', treatIdAsReference:'1', cropVariant: 'smartphone', maxHeight: '{data.imageheight}', width: '{dimensions.width}', maxWidth: '800')}" media="(min-width: 601px)">
<source srcset="{f:uri.image(src:'{file.uid}', treatIdAsReference:'1', cropVariant: 'smartphone', maxHeight: '{data.imageheight}', width: '{dimensions.width}', maxWidth: '600')}" media="(min-width: 401px)">
<source srcset="{f:uri.image(src:'{file.uid}', treatIdAsReference:'1', cropVariant: 'smartphone', maxHeight: '{data.imageheight}', width: '{dimensions.width}', maxWidth: '400')}" media="(min-width: 321px)">
<source srcset="{f:uri.image(src:'{file.uid}', treatIdAsReference:'1', cropVariant: 'smartphone', maxHeight: '{data.imageheight}', width: '{dimensions.width}', maxWidth: '320')}" media="(min-width: 200px)">
<img src="{f:uri.image(src:'{file.uid}', treatIdAsReference:'1', cropVariant: 'default', height: '{dimensions.height}', maxHeight: '{data.imageheight}', width: '{dimensions.width}', maxWidth: '{data.imagewidth}')}"
{f:if(condition: '{data.imagewidth}', then: 'width="{data.imagewidth}"', else: 'width="{dimensions.width}"')}
{f:if(condition: '{data.imageheight}', then: 'height="{data.imageheight}"', else: 'height="{dimensions.height}"')}
{f:if(condition: '{file.title}', then: 'title="{file.title}"')}
{f:if(condition: '{file.alternative}', then: 'alt="{file.alternative}"')} />
</picture>