The name is 500px.com, but what is the image resolution for which to optimize before uploading your photos there?
Photos are displayed at 2048px on their longest edge. Have a read on 500px support page for more details.
From their relevant Support page: width 880px for photos in landscape orientation, height 900px for images in portrait orientation. Even when you submit your images in that size, they will still get resized for display in thumbnails, and on iOS devices. Oh, and your carefully prepared originals in correct size are still recompressed into smaller files.
The name is a leftover from early days of digital photography, when 500 pixels was a suitable size for longer edge of images distributed over slower-than-today connections and displayed on CRT monitors.
Today i figured out why my photos looked like crap. It has something to do with either the time of day you are visiting: every photo will be shown in uber compressed format on certain busy times of the day, or it has to do with how fast your own internet connection is.
I just checked in on 500px, all pictures provide outstanding quality. When I switched back to my satellite internet connection (which has a crappy connection during daytime), everything turned crap again.
so there you have it.