To take a screen capture on the mac's you have to firstly press either CMD-Shift-3 to capture the whole screen or press CMD-Shift-4 to capture part of the screen by dragging the changed mouse over the area you want to grab. After you have captured the screen with one of these commands it will be either on the desktop or in an assigned folder of your choice. To post this picture to blogger you have to firstly log onto blogger, then you have to press post and then you have to click on the button that looks like a picture on the bar above where you can type.
                                                                      This button looks like this. \

Then select the files that you want to upload, click upload and press publish and it will be on your blog.