Wikiwide, hmm, i did not think that someone would select so big image My fault, sorry.
ade, you are right I make this buttons in the begining like test (my interest ), and i was lazy to change it.

Thanks for replies. I will fix all that soon

Thank you. Actually, it was quite difficult to find a bmp image to use, so I opened one of my photographs in X Paint and saved it as bmp. Understandably, the preview of this image was beautiful, but not entirely appropriate as a boot screen. I should have thought to resize and crop it, and then to blend it with Nokia's logo, like Google's doodles... Maybe, later. Either way, thank you for 'unlocking' this part of the device.
Here is the image.
Best wishes.
_________________
Per aspera ad astra...

@raandoom
tried your tool once and everything went fine (changed only background colour to black). Second time I changed image too and tried to reboot, now my N900 is bricked: it doesn`t boot, flasher can`t see it (I can`t cold flash it, or flash it, or flash NOLO only), my windows box can`t see it (so drivers cannot be installed and flasher can`t see it), it doesn`t react to charger.
It`s completely dead - on power button press I can see only white light turn on, but no vibrate, white light turns off after a while and that`s all.

Are there any steps to recover it or I`m fcuked?

artpra, try using linux to flash and/or hold on till windows finds the right driver (or find a way to preinstall them). It can't be that your phone couldn't be cold flashed after changing this. Don't give up...

Even libusb-win32 can`t see it, it`s like bootloader is completely erased.

Bootloader is not erased, otherwise you wouldn't see white light.
This happened to me several times, and I was always able to cold flash it in Linux.
I heard a lot that sometimes flashing bricked N900's in Windows dowsn't work. I don't know why.
Try to flash it in Linux, I'm sure it will do.
UPD: Also, check 0xFFFF.

artpra, don't panic Cold Flashing can make magic. In the last step you can flash only xloader and secondary and all will be ok.
And if you can, give me image that 'brick' your phone. Thanks.

UPD: i tried to flash 24bpp 3 on 3 pixels bmp image just now... and my phone bricked strange. Trying to find reason.
UPD2: 5 minuts and my phone alive again, already tried big 24bpp bmp and all works fine. artpra, i am really need your image.
UPD3: i think i found reason. I delete deb from my post until i fix it.

I will finish it for You: ...if used at Linux box

As You were suggesting to use Linux, I was already at the process of installing Ubuntu - and as You predicted, linux version of flasher connected right away and I was able to cold flash my N900 (after adapting the command to my hardware revision). So I`m back, thanks for Your inputs guys.

@raandoom
I`ve already deleted that image (prior to reading You post), but as I can see You already found what is wrong. Glad that I could help in some way to beta test your "Boot logo changer"

Post updated

@raandoom
bootlogo-changer is a new thread worthy (after all it differs a lot from Xlab tool).
Do it.