I do the following:
1. Open PDF file and go to the page you want to take image from (I'm assuming this will open for you in 'Preview', if not open 'Preview' and then open the PDF file) ... my version of 'Preview' is version 3.0.9, I don't know if this works for older/other versions
2. In 'Preview' go to File then Grab/Selection
3. This will give you a set of cross-hairs that you use to select the area you want, so select the precise area that you want, when you release it immediately opens in a new window, with a file name 'Snapshot ....'
4. Go to File then Save As, type the name you want to save it as in the top box, then go to Format (lower in the Save As frame) and choose JPEG, set the quality as desired, where you want to save it and click Save, you now have a JPEG file saved
5, Job done ... have a well deserved beer/coffee/champagne, whatever is your favourite tipple