How to change my pictures from a NEF file to JPG?

I have a Nikon d3100 which I have taken thousands of pictures with. I recently created a flickr account and am having almost no problems click and dragging photos from my library to upload on flickr's website except for about 15-20 pictures which keep saying "this does not look like a valid photo" and will not upload. I have compared the properties of photos that will not upload to ones that will and the only difference I can find is the photos that will not upload says the type of file is a NEF while the photos which will upload says the type of file is JPG. Also the size of the NEF pictures are over 10mb while the average size of a JPG is around 4mb. Please help for I'm not too savvy when it comes to this type of problem and any help would be appreciated. Thanks again and I hope the solution is not too complicated.

Download paint.net, open them with it, and save as a JPG or PNG file. If they don't open in P.Net, look for an add on on Google that will let you open NEF files with it. Hope this helps!

NEF is a raw formatted image. To convert NEF to JPG, use ViewNX2 to do the conversions. ViewNX2 is on the CD which was packaged with your camera. You can also download ViewNX2 from Nikon's web site. To convert to JPG, use the 'Convert Files' function under Files.

RAW image files are just that. They have to be processed before you can do anything with them.

In Lightroom, we can batch process our RAW files adding keywords, your copyright notice, contact information and any lens calibrations.

Once you process them, you can export them as high quality JEPG files and place them in any unique file folder.

If you do not have Lightroom or another advanced photo program, you may just want to keep shooting JPEG files until you can learn more about how to use photo programs