If your trying to upload using the sites attachment, there can be several reason usually it's because theres a size limit.
Your better off using this method.
This will show how to insert a picture in your post using the "insert image" and a image url.
(I use this method mostly, because using the attachments option has a size limit)
First upload your image to a image hosting service (there are several free one's like postimg.org).
once uploaded copy the "Direct link" of the image location. It will look something like this:
http://s5.postimg.org/m9qqmnxc7/insertimageicon1.jpg
Now when you go to make your post, in the editing window find the "Insert Image" icon and click on it.
this will open the "insert image" dialog
Click on the "from url" tab
Then paste the direct link you copied in the url box
uncheck the "retrieve" box
If everything looks ok, then click ok.
A link similar to this will appear in your post, where ever you had the insert cursor in your post.
[img]http://s5.postimg.org/m9qqmnxc7/insertimageicon1.jpg[/img]
now when you post your message, the picture will appear in your post.
Faster method:
Just type the image tags [img]paste picture url here inbetween these tags[/img]
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