Any image you place onto the document (by drag/drop, File->Place or any other method) will be shown as an 'image' in the Layers panel. If you were to export your document as PDF, we would re-embed the original image data so you would know what's going into your PDF. To draw on an image, it needs to be a pixel layer. You achieved this by rasterising it. A pixel layer is a layer made up of pixels that you are free to manipulate - there is no association to the original image now. When you export the document as PDF you will get an image created in the PDF as per your export setup at the time, so you may end up compressing an image that already shows the compression artefacts of its last save, which is not always what you wanted.
If you log in to the Mac App Store with the same Apple ID that you used when purchasing the original copy of the program, you will find the program on the 'Purchased' tab and you should be able to download it to your MacBook from there.
