Firstly... of course... take your picture and use adobe photoshop to open it. Next resize either the height or width to the specification that u need. Go to Image > Resize > Image Size. Change either the height to 45mm or the width to 35mm.
Next, create a new blank file. Photoshop will ask ya what size u want. Set it to 45mm x 35mm. Make sure the resolution of your picture and the new blank file is exactly the same. After that, go to your original picture... go to layer section, right click your picture and duplicate layer. Choose the destination to your new blank file.
Lastly, go to ur new blank file. Adjust the picture to your satisfaction. Once u are satisfied, then save as a jpeg file. It will ask ya what quality u want. Choose around 6-7 so that it can fit into the requirement of the ICA.
I have sent about 5 applications for the passport thingy and all has been approved and magically younger... wahaha! If u still got any problem just pm me... Btw, i using adobe photoshop element 7... In case other versions got different terms...