Alasdair Taylor's Answer

There are various reasons why this might not be legally permissible.

First, if the work is protected by intellectual property rights (most likely, copyright), and your business does not own the IPR or does not have a licence to use the IPR in this way, then the publication of the work would be an infringement of copyright. Second, if the relationship between your business and the client is confidential, then the publication of the work could amount to a breach of confidence (see https://en.wikipedia.org/wiki/Breach_of_confidence_in_English_law). Third, you should check whether your contract with the client says anything about publicity, as this kind of publication is sometimes a breach of contract.

Even if the publication is lawful, it might be unwise if it annoys an important client! Large brands tend to be particularly sensitive about this sort of thing.