<p>I have a very large background image (about 940x940 pixels) and I'm wondering if anyone has tips for compressing a file this large further than Photoshop can handle?  The best compression without serious loss of quality from Photoshop is PNG 8 (250 KB); does anyone know of a way to compress an image down further than this (maybe compress a PNG after it's been saved)?</p>

<p>I don't normally deal with optimizing images this large, so I was hoping someone would have some pointers.</p>

PNG images are already compressed internally, in a manner that doesn't benefit from more compression much (and may actually expand if you try to compress it).
You can:
Reduce the resolution from 940x940 to something smaller like 470x470.
Reduce the color depth
Compress using a lossy compression tool like JPEG
edit:
Of course 250KB is large for a web background. You might also want to rethink the graphic design that requires this.