This may all be getting a bit techy, but hey, it is better than working (I should be marking student coursework)
I am not sure if this is really a Mac problem or not. You can't start a new topic when you are in an existing topic (such as you are as you read this posting). You have to go up to the main section heading (General Discussion in this case) to see the Start New Topic option. I think that should work, even in a Mac.
Further notes on pictures:
1) If you are using Firefox, if you right click on a picture, one option you get is to View Image. If you click that, it should display the image all by itself.
2) Just below any image you should see its file name, after a little paper clip symbol. If you click on that name, it should open the image in any program you have configured to view that type of image, usually the standard image viewer in Internet Explorer