There's no way to centre an image that isn't a bit hackish, it's not functionality that is provided.
If you really must, you could just precede the image by a load of - or wrap the image in <pre> tags and use normal spaces - to manually add spacing to give the appearance of the image being centred (see this MSO post for examples).
However, this might cause problems for viewers on mobile devices and maybe tablets (not tested, but I just have a suspicion...), so I'd actually advice against this - just leave the images where they are.