You can't copy & paste a picture into a post. TamaTalk posts are text only. When you see a picture in a post, that picture isn't coming from TamaTalk. The picture is hosted on a photo-sharing site. The TamaTalk post contains a link to the picture such that your browser (Internet Explorer, Chrome, etc.) understands that it should combine the text and picture into what you see.
To be able to place a picture in your posts, you first have to upload the picture to a photo-sharing site. There are many free photo-sharing sites: photobucket.com, imageshack.us, etc. Then copy the IMG link code for that picture. There are several different types of link codes. The one you want is "for bulletin boards & forums". Finally, when writing your post on TamaTalk, paste the IMG link code where you want the picture to appear in your post.