Although BootLogoCustomizer doesn’t allow you to change the boot logo that appears when you first reboot your iPhone, it will let you change the one that appears on respring, and it will eventually show up on reboot after the unmodified logo is displayed for a few seconds.
After installation of the tweak, venture into the stock Settings and find BootLogoCustomizer’s preferences. Inside, you’ll see a kill-switch, two sections for customizing the boot logo color and boot logo background color, and an Apply Changes button, which serves as a respring shortcut.
As I previously alluded to, this tweak doesn’t actually allow you to change the real boot logo, since that loads outside of Cydia Substrate’s jurisdiction. It does, however, allow you to change the boot logo on respring, and will present itself briefly on an actual reboot once Substrate is loaded into memory.
If you’re looking to spruce up the respring process, then I can’t think of a better tweak to do so with. BootLogoCustomizer is available for $1.00 on the BigBoss repo.