# Context
We use `ImageServiceRaster` with `RasterLayer` along with the MosaicRule
# Issue
There is no way to provide `format` parameter which leads to ImageServer request with "lerc" format, which causes transparency issues. The request path & query is smth like: /ImageServer/exportImage?format=lerc&...
# Request
Provide `format` or `preferredImageFormat` (similar to WmtsLayer) so we could decide which tile format we want to obtain from the image server.
