Yep, let's wait and see. Probably, that other image (I read from your posts - with difficulty - that you already gave a try with another, smaller image) *was* indeed a kernel. About 2-3 MB size: likely kernel. About 650MB size: likely root filesystem image.
When booting up, I think the kernel will flash the green power LED once. So if it flashes, the kernel is intact. If the loading bar appears, the root filesystem is in parts intact, if it never succeeds, the root filesystem might be corrupted.
So: the problem you had first is likely a corrupted root filesystem, partly intact.
When you flashed a kernel (or another image) to the root filesystem offset, that changed into an invalid root fs (not even partly intact), thus no progress bar would appear.
Go on and try flashing that large image, then report back :-) Remember that this will likely need a battery that is charged enough to last for ~2 hours. For me, flashing the root filesystem was very energy efficient, I think it might have taken about 10% maximum from the battery (if at all, I might have been wasting them on other things).