Yes, pics do usually have a 3:2 ratio, but don't take pics that don't & stretch them to make them that ratio - that just fucks up the pics & makes them look like shit.
I think your problem is that you're set on making one side 500 so that the pics fit the minimum for most link lists - so then you end up with over compressed pics that look like shit just to save $1 each month on your bandwidth
Where is this particular content from & what are the original dimensions?