I did an illustration in Photoshop, all with Shape Layers only. All the lines are smooth and perfect within the working file. It's perfect when I export it into a png or jpg also. Problem arises when I place that exported image into another psd document. The (smooth)lines now appear jaggedy(pictured below). Note that this happens @ 100% zoom. And, the placed image is of much higher resolution than the document in which it is placed.
The original artwork was done in A3 size @ 300ppi. The receiving document is a relatively low res 900px wide @72ppi. I have a hunch that the problem is because of this. Can you help me find out what the problem is?