To open local raster files in the SDK, you'll generally need a couple of support files. A world file to describe the image-to-world transformation, and an aux file to specify a spatial reference, so that the SDK knows how to handle that georeferenced image file.
See this gist for an example. The links at the bottom talk about the world and aux files, as well as what to name them. I've found that just using the aux file from the gist wholesale (with an appropriate name) and writing out an appropriate world file based off the instructions in the links, and placing them alongside the image file on device storage works well.
This will require a Standard license or above, but if you're using Local Server, you're already there (and of course while you're developing, without applying any license, you can also test this out). You should also be able to use Local Server for serving up rasters, but if you can get the aux and world files working that might be a simpler approach.
