But let's imagine that we built a blockchain based on the usage and the processing of images, videos and documents for digital forensic purposes; what would a suitable block size be for that, if we're looking at a consortium-type blockchain?
If I understand this correctly, then if the block size is too small (less than 1MB), it would cause an image (let's say 100MB in size) to split/fragment? Or would the blockchain simply ignore any and all data of such great sizes?
It depends on the consensus rules of your network. Based on Bitcoin's rules, a transaction so large would be rejected by the network completely because it's too large to fit into a block. You would need to manually split the data into small enough transactions (<1MB each) and then pay the appropriate fees for each transaction.
If the block size is too big (let's say 1GB), would I need to fill it up with ten images of this size (100MB) before it's appended to the blockchain?
In Bitcoin (and virtually all blockchains), the block size (or block weight) limit is just an
upper bound. Miners can publish empty or partially-empty blocks if there aren't sufficient transactions to make full blocks.