To be honest, I think you'll be fine upto 1024 on it's longest axis, but I've never uploaded one that big, so I can't swear it. Photopost can be very picky about resolution, image demensions, etc...
nowq why cant i upload any pic. they are less then 800x600 and the file size is less the 80 kb. but it just goes to a all white screen after i click the upload button.
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A 46 gal bow front (Soft coral) reef tank with a 10 gal sump. And a 30 gal (SPS and Clam) reef tank hooked up to the sump of the 46 so they share water.