This is how I did it. First, I went on my computer (this really helps) second, I went to Flipnote Hatena mobile (http://flipnote.hatena.com/mobile/) and clicked on the flipnote I wanted to use. Third, I right clicked on the flipnote and selected "open link in new window". Fourth, I copied the address and pasted it in the comment with the tag [ image ] in front and [ /image ] at back.
You can insert just about any image that has a url. On most PC browsers, when you right-click an image, there will be an option for copying the image url. (It's not necessary to be on Hatena mobile, by the way.)