1) Create your custom loading.bmp and exiting.bmp.
Both must be exactly (in pixels) of the size of your screen resolution (800x480, 480x272, 320x240, etc). You can use any graphics editor you like, but the Resulting images must be Precisely sized to your resolution size, be in 24-bit color depth, and saved as standard "Windows" bmp format. KB in size, the images must be exactly this: 320x240 = 225 KB, 480x272 = 382 KB, 800x480 = 1.09 MB.
If, after creating / editing / saving, your files are not These KB sizes, you have done something wrong and must start again.

2) Rename your saved loading.bmp this loading.jpg ... note: do NOT convert the image to JPG, just rename the file!
2a) Rename your saved exiting.bmp this loading.bmp.
I can not get any success with the instructions above.
My phone has 320x480 resolution. So I guess 480x272 pictures won't work, right?
Also, renaming loading.bmp to loading.jpg.bmp, or just loading.jpg?
In sys.txt file to add lines with quotation marks or without them?
[Loading]
loading_bmp = "loading.jpg"
And these two files must be placed in data.zip archive, right?

Thank you.