Did you have a verbal or written contract? The contract must state the payment terms otherwise the company may wriggle off the hook by stating in court that their terms are Nett 180 or even Nett 365 days.
If there's no written contract and they have used your images and not paid for them then they are in direct breach of Both the Berne Convention on Copyright 1971 and Amendments and also the DMCA. This will allow you to serve notice on the company that they have X days to pay you a stipulated price per image, or remove and destroy all copies of the image in their possession. The notice is best served either by courier or registered mail and should include not only your full contact details but also a reference number, the date and so in. Use only formal business English.
How large were the images in the magazine? If you can specifiy in column widths it will be possible to work out an appropriate charge per image. Obvioiusly unless you are a known quantity as a professional photographer you won't be able to charge several grand per image, but depending on the material and size of the prints in the magazine you may be able to charge upwards of 1,000 SGD in total. (note I said maybe).
I'd suggest giving them 14 days to respond to your letter of demand. After that file for copyright infringement, unauthorised publication of copyright material and go after them for punative damages. (great fun that one). Do NOT blacken the magazines name online or in a blogg etc until the matter is resolved in court.