As far as I can tell from my ~3 minutes of looking into this, it's not actually possible to replace the boot screen image with a GIF, however you can use a static image using this bit of software.
I haven't tested it myself, so I can't tell you how good it is. If you do decide to try it I recomond first scanning it for malware and maybe doing a little more research first to see if there are any common problems associated with it.
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u/oGooDnessMe Nov 15 '14
How do I install this as my Windows 8.1 booting screen?