When you're using the online editor, there are alignment guides that will assist you with aligning images. When re-sizing or moving an image, as the edge of the image comes into alignment with another image, the alignment guides will turn green to show you that the image edges are aligned.
If you centre an image in a page, as you move the image into the centre of the page green centre lines will appear to show that it is centred.
On bigger products, larger books, canvases, etc. It's best to zoom in when aligning. That way you have a much finer control over how accurately the images are aligned.
The alignment guides are there to help you know when the images are aligned, they do not snap the image into alignment, the accuracy of the alignment is ultimately up to how carefully you adjust the images. Zooming in to get a good alignment is highly recommended.