Adds an image to be used in the style. This API can also be used for updating an image. If the image for a given `imageId` was already added, it gets replaced by the new image. The image can be used in [`icon-image`](https://www.mapbox.com/mapbox-gl-js/style-spec/#layout-symbol-icon-image), [`fill-pattern`](https://www.mapbox.com/mapbox-gl-js/style-spec/#paint-fill-fill-pattern), [`line-pattern`](https://www.mapbox.com/mapbox-gl-js/style-spec/#paint-line-line-pattern) and [`text-field`](https://www.mapbox.com/mapbox-gl-js/style-spec/#layout-symbol-text-field) properties.
Return
A string describing an error if the operation was not successful, empty otherwise.
Parameters
content
An array of four numbers, with the first two specifying the left, top corner, and the last two specifying the right, bottom corner. If present, and if the icon uses icon-text-fit, the symbol's text will be fit inside the content box.
image
A pixel data of the image.
imageId
An identifier of the image.
scale
A scale factor for the image.
sdf
An option to treat whether image is SDF(signed distance field) or not.
stretchX
An array of two-element arrays, consisting of two numbers that represent the from position and the to position of areas that can be stretched horizontally.
stretchY
An array of two-element arrays, consisting of two numbers that represent the from position and the to position of areas that can be stretched vertically.