Attachments doesn't work the best here, the best bet is to upload your pictures to a third party such as Flickr, Dropbox or anywhere really you can get an image URL. Once you have it it should look something like
http://www.dro..................jpg If you have a picture already online you want to use right click it and view image URL. Once you have the image URL you type it in between the [image] tags found above the textbox your typing in.