Conditional selection of image and returning a reference if the _type is image, otherwise returning the rest of the available fields.

content[]{
  _type == 'image' => {
    altText,
	caption,
	"image": asset->url
  },
  // Return the rest of the fields without condition
},

content[]{
  ...,
  _type == 'image' => {
    altText,
	caption,
	"image": asset->url
  },
},