I believe you also need to use either genfb or radeonfb. The splash screen is also very picky about the image format[1].
All of the NetBSD systems I have right now are headless, so I built a kernel in Virtualbox and got the splash screen to load a gif[2] by changing the following in GENERIC
$ diff -u GENERIC MYGENERIC
--- GENERIC 2019-03-03 07:11:50.989103363 +0000
+++ MYGENERIC 2019-04-11 08:28:45.739640931 +0000
@@ -280,7 +280,7 @@
# enable VGA raster mode capable of displaying multilingual text on console
#options VGA_RASTERCONSOLE
# enable splash screen support; requires genfb or radeonfb
-#options SPLASHSCREEN
+options SPLASHSCREEN
# Kernel root file system and dump configuration.
config netbsd root on ? type ?
@@ -411,10 +411,11 @@
options PMS_SYNAPTICS_TOUCHPAD # Enable support for Synaptics Touchpads
options PMS_ELANTECH_TOUCHPAD # Enable support for Elantech Touchpads
options PMS_ALPS_TOUCHPAD # Enable support for Alps Touchpads
-vga* at pci? dev ? function ?
+#vga* at pci? dev ? function ?
genfb* at pci? dev ? function ?
options VCONS_DRAW_INTR
-wsdisplay* at vga? console ?
+wsdisplay* at genfb?
+#wsdisplay* at vga? console ?
wsdisplay* at wsemuldisplaydev?
wskbd* at pckbd? console ?
wsmouse* at pms? mux 0
Interestingly, the splash screen never seems to go away until I reboot the system. I think there might be something you have to do to clear it, maybe via rc.d?