Problem: I wish to store the images in a prepared sprite like - "Load from URI(Sprite.CurrentImageAsURI)"
Why'd you like to store it in a prepared sprite? Is there a reason you cannot use an ajax request to load the image? If you only want to load the image once, you can store it inside a binary data object and load it from there.
And for changing a variable to change the theme, that's also possible with this approach. You can request a URL with ajax rather than a file, that you can point to the local resources by string and just add the global variable in the name. So you'd request "stringsub("./tiledfile{0}.png", globalVar)
This would request e.g. tiledfile1.png or tiledfile2.png or tiledfileVolcanotheme.png, etc.
I wouldn't attempt approach 2.