Make loadBg() more readable

[w00000dy]

In my quest to comprehend the functionality of the loadBg() function, I initially found it challenging. To facilitate my understanding, I annotated the code with comments and renamed variables for clarity. I would now like to share these modifications, hoping to simplify the code-reading process for future readers.

I also made some small improvements.
For example, the line img = null; is not strictly necessary.
The line img = null; is used to remove the reference to the img object, which can potentially allow the img object to be garbage collected sooner if there are no other references to it. However, in this case, the img object is created within the function and is not used outside of the function, so it will be garbage collected anyway once the function has finished executing.
This is why I removed this line.

Because of this change and the other small changes, we also saved a few bytes:
Before:

RAM:   [==        ]  24.7% (used 80968 bytes from 327680 bytes)
Flash: [========  ]  82.3% (used 1294253 bytes from 1572864 bytes)

Now:

RAM:   [==        ]  24.7% (used 80968 bytes from 327680 bytes)
Flash: [========  ]  82.3% (used 1294205 bytes from 1572864 bytes)