I did 850 x 315 @ 300dpi
but when its uploaded - it looks wierd:
There are a few things that could be causing the problem. 300dpi is for print, on screen works to 72 dpi so changing the image to that could work for you (make sure resample image is not ticked) Another problem could be the way you have saved the file, try using the save to web option rather than just the normal save. This will optimize your image for web. Sometimes the image can have to many shades of a colour and there for the quality gets reduced, you can control this within the save for web option. Also make sure your colour mode is set to RGB.