Oh, this is fun! Story time.
Lets say the owner of the bridge actually gave permission to the graffiti artist to create the 'mural' in the first place and said artist agreed to let others modify said mural as long as credit was given to original artist.....
Artist A adds a highlight to the eye of the tiger (by Fishy 123) in the mural and then signs it Brutus248. (Fishy123 didn't sign it, btw) Photographer take a picture of the eye of the tiger and gives credit to Brutus248 in their report to the Hippity Hop Art News.
Fishy123 then contacts the village people about taking Brutus248's siggy off the eye of the tiger.
To Be Continued....
If the owner of the bridge gave permission, it ruins the whole story, then we are off onto a whole different topic.
What is more intriguing is the fact that copyright law is not the same in every country, and N3V is in Australia, and I am in the USA, and clam1952 is in the UK. So who's laws do we abide by? For me it must be the N3V location. so here is what the Aussies laws state, pay attention to the last sentence;
Fair dealing
The main exceptions to copyright infringement in Australia come under the general heading fair dealing. Fair dealing is comparable to the United States' fair use; it is a use of a work specifically recognised as not being a copyright violation. In order to be a fair dealing under Australian law a use must fall within a range of specific purposes. These purposes vary by type of work, but the possibilities are:
review or criticism
research or study
news-reporting
judicial proceedings or professional legal advice
parody or satire (added by the Copyright Amendment Act 2006)
In order for a certain use to be a fair dealing, it must fall within one of these purposes and must also be 'fair'. What is fair will depend on all the circumstances, including the nature of the work, the nature of the use and the effect of the use on any commercial market for the work.
Fair dealing is not the same as fair use. This has, for example, been interpreted by US courts to allow for reasonable personal use of works, e.g. media-shifting, which would not necessarily be permitted under Australia's fair dealing laws. Australian copyright law does, however, have a number of additional specific exceptions which permit uses which may fall outside of both fair dealing and fair use. For example, a number of exceptions exist which permit specific uses of computer software.