fixes https://github.com/TryGhost/Ghost/issues/14323
- Fixed support for resizing images from Unsplash using the `img-url` helper (previously the size property was ignored for images from Unsplash)
- Added support for `avif` file formats (supported by sharp out of the box)
- Added support for setting the format of images, with a new `format` option:
E.g. to convert an image to webp (only works in combination with size for now, except for Unsplash where you can use it without size):
```
{{img_url @site.cover_image size="s" format="webp"}}
```
This can help improve the performance of a theme, by serving assets in `<picture>` elements with webp and fallback image formats.
Usage example:
```html
<picture>
<source
srcset="{{img_url feature_image size="s" format="avif"}} 300w,
{{img_url feature_image size="m" format="avif"}} 600w,
{{img_url feature_image size="l" format="avif"}} 1000w,
{{img_url feature_image size="xl" format="avif"}} 2000w"
sizes="(min-width: 1400px) 1400px, 92vw"
type="image/avif"
>
<source
srcset="{{img_url feature_image size="s" format="webp"}} 300w,
{{img_url feature_image size="m" format="webp"}} 600w,
{{img_url feature_image size="l" format="webp"}} 1000w,
{{img_url feature_image size="xl" format="webp"}} 2000w"
sizes="(min-width: 1400px) 1400px, 92vw"
type="image/webp"
>
<img
srcset="{{img_url feature_image size="s"}} 300w,
{{img_url feature_image size="m"}} 600w,
{{img_url feature_image size="l"}} 1000w,
{{img_url feature_image size="xl"}} 2000w"
sizes="(min-width: 1400px) 1400px, 92vw"
src="{{img_url feature_image size="xl"}}"
alt="{{#if feature_image_alt}}{{feature_image_alt}}{{else}}{{title}}{{/if}}"
>
</picture>
```
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