NOTE: This is not an answer to the question, but a description of some of my findings
From what I can tell, the files are a custom archive format and they're probably compressed using DEFLATE since compressing them using ZIP didn't change the filesizes much while compressing using RAR, did.
Each file looks like it starts with the file signature 32 30 53 52
followed by what looks like a listing of the files contained within the archive. 42 bytes after each file name string looks like they might contain information about how the files are stored in the structure.
32 30 53 52 09 00 00 00 74 61 62 6C 65 31 2E 70 20SR....table1.p 6E 67 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ng̲.̲.̲.̲.̲.̲.̲.̲.̲.̲.̲.̲.̲.̲. 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ̲.̲.̲.̲.̲.̲.̲.̲.̲.̲.̲.̲.̲.̲.̲.̲. BD 12 03 00 3C 04 03 00 DC 01 00 00 74 61 62 6C ̲½̲.̲.̲.<̲.̲.̲.̲Ü̲.̲.̲.tabl 65 32 2E 70 6E 67 00 00 00 00 00 00 00 00 00 00 e2.png̲.̲.̲.̲.̲.̲.̲.̲.̲.̲. 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ̲.̲.̲.̲.̲.̲.̲.̲.̲.̲.̲.̲.̲.̲.̲.̲. 00 00 00 00 BD 12 03 00 3C 04 03 00 18 06 03 00 ̲.̲.̲.̲.̲½̲.̲.̲.<̲.̲.̲.̲.̲.̲.̲.
You might get a better answer at Reverse Engineering.
EDIT: I found out later that 42 bytes are not always used to (possibly) represent information about the archived file since the length of the zeroes preceding the non zero bytes varies from one IMG file to another.
However, I found out that 12 bytes preceding a filename string are always non-zero (ending with a null byte). In the above example, they'd be BD 12 03 00 3C 04 03 00 DC 01 00 00
and BD 12 03 00 3C 04 03 00 18 06 03 00
respectively.
As an alternative solution, I'd suggest that you use a software program like f.lux that attempts to adjust the color temperature of the computer's display according to the time of day to make it easier on the eyes.