Some of the links to posting pics are a litle out of date but the add/edit page is correct. To get there click on the "My Pictures " link at the top of the CMoN home page.
Once there fill in the data regarding your pic, and choose the method to upload it - either direct from a website or from your computer. If you link from a website the image will stay on the system as long as the website is running and the URL is not altered. Change either and the pic won't show up ( anyone feel free to correct me if this is wrong). If you upload a pic from your computer then it will remain forever on CMoN unless you actively remove it.
Info on the pic - images are shown on the browse screen at 600 pixels wide, but you can magnify images up to 1200 pixels wide. So it's best to get the correct width before you upload the images or you will find it being compressed which might make the image a lower quality than you had expected. Likewise the max file size for uploading is 1024k (which to be honest is more than enough for most images, even multiple pic images); over do the file size and the image will again be compressed resulting in lower quality.
Here's a useful link to an article regarding multiple pics and general posting etiquette -
http://coolminiornot.com/article/aid/587
I hope that helps, and good luck with your images
