You probably have too big a file. Reduce the largest side to no bigger than 2000 pixels and if you are using a bicubic method of sizing, the other side will automatically follow. You should save the photo as a tiff or gif file to upload to the web and a jpeg file if you want it to be easily printed from a number of sites or programs. Not all formats are recognised by different photo processing programs but those three are fairly universal. Good luck next time.
P.S. make sure you duplicate the image before you reduce it so that you still have the original intact. Find where your program tells you the size of the image and work from there.