The "size" of an image on here is the number of bytes of data stored in the image.
NS has a limit of 100kb, which is 102,400 bytes (1kb = 1024 bytes).  For an image that is about as wide as the NS content area (from the edge of the "online" bar to the edge of the "quick links" bar, you'll have a tough time compressing a file to be under 100kb.
If you can, get an image program that'd let you resize your image to approximately 550 pixels wide (or tall if it's a vertical photo), and compress the file when saving it so it is under 100kb.  Shouldn't be hard, I've never had a problem getting images to fit on here.  No one did back when the limit was 30kb either.