Marjo,
As a former free-lance writer, and a (retired) lawyer who has worked with free-lancers over issues like this in the past, I'd like to point out, that while there are laws that govern contracts, what governs most contract issues is the contract itself. It is also important to note that there are two basic ways that free-lancers get paid, one is upon acceptance, and the other is upon publication. The contract you have with the magazine should cover this issue. In working for a number of publishers, the signed contract was treated as the bill (whether in acceptance or upon publication agreements). In that regard, requiring an invoice of you seems like a stalling tactic - if they pay on publication - they are the only party that knows the actual distribution date. You shouldn't be expected to be scouring the newsstands to see when you can send in an invoice.
What is unclear in your post is, how long has it been since; you gave the magazine the photos, how long has it been since they were published, were they published online and were you properly credited?
If you haven't already done so, you might want to go to the website of the American Society of Media Photographers at:
http://www.asmp.org/If you want to, you can contact me privately at mitchmberger@msn.com.
As an aisde, years ago, a friend of mine, tired of waiting for payment from a publisher who always paid late, went to their offiecs after the issue with her work was on sale, and waited in their lobby and ate a giant (and quite fragrant) tuna salad sandwich until they cut her a check.
masguy