This might work for other Butterfly devices as well, but I'm not 100% sure.
As a test, I found a random splash screen, resized it to 1080x1920, converted and flashed it using EZ Splash Screen Flasher, and bam, changed splash screen. The htc development message is still there, though.
Here are some images I used:
HTC Anime Girl
The above images were found in the guide I linked above, so original credit goes to whoever provided those images. I'd be happy to credit them here if I find who they are.
If you don't want to go through the process of creating your own splash screen, you can download the attached zips with the img file inside, put the img file somewhere you can use the fastboot command. You'll need to put your phone in fastboot and use the command
fastboot flash splash1 splash.img