Finally i finished it At a snail's pace, step by step, but GUI for changing boot logo was finished.

Yes, a lot of time has passed and maybe it is too late for this application, but i gained experience and maybe someone be happy

Now there is no need to carefully choose images and compress it with elf, and then count its sizes, and... No need for that.
All what you need for changing your boot screen is:

1. Choose background color
You can choose color with 4 ways (why not? ):
a) write 24-bit color in dec, like this: [180] [180] [180]
b) write 24-bit color in hex, like this: 0xAABBCC
c) write 16-bit color in hex, like this: 0xABCD
d) using standart ColorDialog from Qt

2. Choose images
Images must be BMP images with 16-bit or 24-bit color.
Don't worry, if you choose wrong image - application let you know about this

3. Preview
You can see how your boot screen will looks like by pressing "Preview".

4. Applying changes
For applying changes you need to press "Apply". This will open log window.
Before bootloader will be patched application need to be sure that chosen images can be successfully written to bootloader (limits for compressed image size can't be canceled). You need to press "Check" for checking all, after this some information about chosen options will be printed to log. If application find some errors, it will not allow you to flash your bootloader before errors reasons will be fixed.
If all test was successfuly passed, you can flash your images and bootloader by pressing "Patch".

Next, you must wait a few seconds for message about successful flashing, and relax

By the way, you don't need to patch all options in one time. You can choose what you want to patch by clicking checkboxes in top left corner.

p.s. Icon a little ugly, but it is icon

UPD: package returned to post. About allowed image width and height: bootloader has something, that don't work correct with some image sizes. I cant find a clear pattern for incorrect image width and height.

But, just in case, i add limits for minimal image size (width and height): if image has width and/or height less than default image, application will not allow you to select this image.
For fan of experiments this option can be disabled

Current version: 1.0.1
Changes:
- fixed GUI blinking
- left buttons states changed from enabled/disabled to checked/unchecked
- and something important: added action to menu bar for disabling image size limitations

p.s.: you can take a look on text file with logo sizes that i tried, with the results. Maybe someone will find pattern for incorrect image width and height
And one more: maybe will be better if you post your results (false or success) with image
sizes, but only if your image has width or height less than default.
Thanks.