Dear sunny,
please follow the below instructions exactly other wise you will face the problems again and again, i was having same trouble till i follow these instructions noting that navtwaek is great for editing the navboot but not the logo screen, you have to use gifextractor for this issue.now follow these.
1- be sure the firmware is in .iso format using either magic iso,ultraiso,poweriso,etc..
2- You will need the below programs and please only these programs
2.1 - Gifextractor download = http://www.efilament.ae/downloads/GifExtractor.exe
2.2- ultra iso download = http://www.carmes.de/download/downlo...fExtractor.exe
2.3 - .net libraries download = http://www.microsoft.com/downloads/d...displaylang=en

3- install all of the above.
4- open ultra iso then in the below right screen locate the .iso ( firmware ) and drag it to the screen above it ( screen is like a square)
when u will drag it it will show all files in this .iso image.
5- when all files will show Locate "navboot" file. Usually located in \V_2\RR\0101\BMWC01S\app_sw\
6- when u locate it, right click on it the choose extract to desktop.
7- when u extract to desktop,open gifextractor, then tick the three box's which you will see below in the end of the program which is ( correcr crc24,read complicated gif, and dont check file size )
8-in the top of the program you will find icon ( find file ) press it and locate the navboot file in the desktop then press open
9- then becide the ( find file 0 there is a bottom (extract ) press it
10- now, the program extract many files , press on each of them till you find the image ( =====BMW=====)
11- go down becide the three box's there is a box called selct image to exchange
12- now select the .gif image and press open .NOTE ( image must be .gif, single frame, 8 bit, 400x234 and the smaller the better but its tested with my image which is 36 KB and its working perfectly.
13- after you select the image close the program and run it again 9 gifextractor )
14- again do the above steps to check wether the image is loaded or not, dont exchange the image again, just check if its loaded then close the program, and be sure every time you start the program click the three box's before extracting the file so it can read your imahe.
15-now go back to the ultraiso which you open in the begenning
and in the same folder which you extract the navboot from press f3 then select the modifed navboot in the desktop and select it , you will be promoted to overwrite, for sure press yes.
16- now you are ready to burn, press f7 and then locate the image file and burn it, this wil work then.
for any other info or help im ready.
thanks. good luck