Wait...what...you're using photographs that are in another location, on some other web site?
Just taking a shot here, but all your clips, and all your photographs--everything--should be in the same folder on your own web site.
There's more: Never mind the 16:9 ratio, the photographs and the clips must be the same size, right down to the pixel. If not, then JW Player would have to do an on-the-fly conversion (if there is such a thing), and this would sure enough slow things down. A lot.
Sorry if I've misunderstood your problem, and if I have, then I have no further ideas. But this is the way I understand your situation.