Considering your pictures are so beautiful and airey, I'd go for or at least try a white or lighter color background to compliment those pictures, and I'm just not liking the pictures not showing up full screen, however that in itself presents a whole new problem. Landscape orientation would work great, but you would be cropping pictures that are in portrait mode in order to 'fit-to-screen' them.
I guess then it comes down to gauging which ones are majority , landscape or portrait.
And yes, the site is a bit slow to load. I doubt that's got to do with the size of the pictures, but more so with the 500px system.
Off the topic, how much are you paying for this monthly/annually?
And have you tried photoshelter or any other similar websites?
Also. I'd lose the background picture, I think its distracting from the beauty that stands before you. A plain background would however bring all the attention to your pictures.
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