This whole preload image business was added without my approval a long time ago, and I never liked it.  It wasn't doing us any good because preloading images makes sense when there is a slow network and a fast CPU, when load time is much longer than decode time. But on a mobile device with local images, load time is fast and decode time is long.  So preloading doesn't do any good because the image still needs to be decoded.

Also, I think the preload image element may have been contributing to the OOMs.  I think (but have not written tests to confirm) that adding #-moz-samplesize to our URLs prevented gecko from sharing image memory between the preload image and the on-screen image.