To enlarge on Malcolm's questions, can you see the extension for the image? If it says .bmp or .tif, it is the original, uncompressed size.

That is almost never needed for online viewing. Any of the programs mentioned will give you options for reducing images to a reasonable size by converting them to .jpg. (the most common format)

Usually a size of 800X600 or above...up to 1200X1000 or so will do.
You can make several copies in various sizes (saved with slightly different names)and decide which looks best.