add
Adds an image to be used in the style. This API can also be used for updating an image. If the image \a id was already added, it gets replaced by the new image.
The image can be used in icon-image, fill-pattern, and line-pattern.
Return
A string describing an error if the operation was not successful, empty otherwise.
Parameters
imageId
ID of the image.
bitmap
Bitmap data of the image.