Two ways...
1) Create a bitmap file with the same name as the database but with a .BMP extension. The bitmap will load as the database is opening and then disapear. If you are using Access security, the bitmap will display as a background until the user logs in.
2) Create a form and load it as the "Startup" form. Tools > Startup... > Display form. Then use the timer function in the startup form to make that form close after 5 seconds and open another form.
HTH