An icon from a real DS game (on the whitelist) is needed to circumvent the anti-flashcart checks on DSi and 2|3DS consoles.
* It is
NOT part of the kernel/software on flashcart micro-SD
* Cannot be changed to an arbitrary picture (DSi and 2|3DS won't accept them)
The availability of different icons depends on what updates the manufacturer had built in the past. For Acekard 2i there are quite some icons available. The first reply in the following thread also is about a Spongebob icon; maybe your cart is something different
and not an Acekard 2i.
https://gbatemp.net/threads/restoring-old-headers-for-the-acekard-2i.482115/
!!
Downgrading the flashcart firmware is risky and can lock it out (blacklisted) – if your console(s) do(es) not have custom firmware installed !!
My advise: If it's not broken – don't try to fix it.