Consistency is important to the reader, too. If the transcriber has been using box lines for the material prior to this point in the transcription, she should continue to do so. For the print graphic you attached, an open box line would be followed by the centered heading and then a blank line. I would then move the description of the prokaryotic cell [u]above[/u] a simplified version of that cell illustration (with a blank line between the text and the graphic). The tactile would be followed by a another blank line and the descriptive text for the eukaryotic cell, a blank line, and a simplified version of that cell. You would then need another blank line before the closing box line. Guidelines and Standards 5.1.2 requires a blank line before and after a graphic, even though Braille Formats would say that you cannot have one before a closing box line. It is doubtful whether all of this information would fit on one tactile graphic page, even with simplified graphics, so I would probably place the title and left side of the print graphic on the first page, followed by the right side of the print graphic on the next page. The closing box line would therefore be on the 2nd page.
Please note that there do not seem to be any examples in Guidelines and Standards where boxing lines are used. Please refer to the Counting Symbols example on page
6-62 through 6-64 where it could have been argued that boxing lines should be used. However, two of the key points made in Guidelines and Standards are to simplify, and reduce clutter.