Assuming you rebuilt properly rebuilt the kernel.img with the logo, here are a few things to check:

- if your firmware boot.img contains the kernel, you should split it to remove the kernel (use scripts like split_bootimg.pl) and make sure boot.img contains only the ramdisk. Once it's done, you need to sign this new boot image (use tools like rkcrc -k) and make sure you also flash it in addition to your rebuilt kernel.img.

- if you rebuilt your firmware in update.img style and flash with Rockchip Batch Tool, you need to make sure that the kernel line is not commented (line showing as as #kernel) in the package-file.

So it should read:

kernel          Image/kernel.img

Cheers.