Image resize / conversion

How can I resize a picture and save it to bmp (which is the only supported format by navision?)?

I've constructed a .NET dll which can do the trick, but I can't seem to register it in Navision. This code has also been compiled to a .exe file, but using shell() forces a short popup which is not wanted...

So I either need another solution for the resize of the image, or I need to know how to register my dll.