Even if the obviousness of the basic idea is not enough to throw this out there should be plenty of prior art available [...]
There is no such thing as "obviousness of the basic idea" in the absence of relevant prior art. I do not mean to say that this specific patent application is necessarily non-obvious, just that that's not how you determine obviousness in the context of patent law.
[T]here should be plenty of prior art available for calculating checksums for each color plane in various old picture and video compression standards, container standards and open source software [...]
That may be, but I wouldn't bet on it. Why would you want to calculate a separate checksum for each color plane? Certainly, it's likely that some lossless image compression scheme can be found that involved calculating a checksum for a whole image to ensure proper decoding. But why do it separately for each channel?
Indeed, I'd say there's no clear utility for the patent, and that might be a good rationale for attacking it without looking too hard at the prior art.
The specification does mention that "[t]his checksum may be used by the decoder for compliance testing, bit stream transmission or storage error detection, etc" but it is questionable whether that is a specific utility. Any other method of transmitting checksums that covers all of the data achieves these goals equally well.