I am thinking of modifying phpBB for a website. If I am asked to provide my modifications am I required to do so. According to the FAQs it looks like if the site is accessed by the public I do and if it is not, only for internal use, I don't. Also does the license only cover the code or does it also cover images and such added to the site as well. Can anyone clarify here is the quote from the FAQs:

A company is running a modified version of a GPL'ed program on a web site. Does the GPL say they must release their modified sources?

The GPL permits anyone to make a modified version and use it without ever distributing it to others. What this company is doing is a special case of that. Therefore, the company does not have to release the modified sources. It is essential for people to have the freedom to make modifications and use them privately, without ever publishing those modifications. However, putting the program on a server machine for the public to talk to is hardly "private" use, so it would be legitimate to require release of the source code in that special case.

You can keep using the modified version of the software as long as you do not intend to distribute the modified version but only sell its services. But people's opinion wildly differs on this point.

But you stand to gain a lot of respect and friendship if you choose to redistribute under the terms of GPL to the letter. This is not a necessity and you don't have to submit patches if the development model is unfriendly or such reasons. But shown your modifications, the developers might change mind in order to accommodate your wishes in order to apply your patches. This happens only after you gain a good deal of credibility.