Please be aware that changing the file could result in an unstartable Maya, so
PLEASE BACKUP THE FILE!
If all went wrong and you don't have a backup, take your Maya installation CD and open the cab-files with WinRAR. Extract the file...
First of all download a program like eXescope or restorator ( Link for Demo! / or any similar resource editor). Install it and start it. Than choose File / Open and point it /Aw/Maya4.0/bin/MayaRes.dll. Choose the + to advance the resource section. Once again for Bitmap. Than go down the List to MAYASTARTUPIMAGE.XPM. Choose file export and save the file as "Maya_Default_Startup.bmp". Make a copy of that file and bring your work in it. The resolution has to be 600*420. It MUST HAVE THE SAME SIZE!!!! You're on the secure side if you use a copy of the default one and copy your image in it!
You should make a copy of your MayaRes.dll. Name it for example ..._default or so... Than use eXescope again as before and choose Import. Point it to your new Image! Press the update Button! Or Save/Update. Say yes to the question. If another dialog comes like the data changes or something like that, don't choose yes! The dll will be corrupted!!! As said before, choose a copy of the default image and change it, copy your image over it...
Have a look at my window: Maya Startupscreen (choose save link!)