You can use the Preview application that came with your iMac to open PDF files, and then export them to JPEG, TIFF, or PNG formats.
1) Open your PDF document in Preview, then scroll to the page that you want to export.
2) Pull down the File menu and select Export.
3) Select the desired format from the pull down Format menu.
4) Click on the Save button.
5) Repeat for other desired pages.
I think using the 'Automator' is going to be the answer if you have a lot of files to convert. Here are some links that may be helpful.
Here's an excerpt from a different Blog:
The best fix I found is an automator script. Open Automator and use 3 scripts, ‘Render PDF pages as JPEG’, ‘Copy Finder Items’, and ‘Make Sequential.’
I have never done an Automator script, but it is easy. The result is an Automator icon I labeled as ‘Multi Page PDFs to JPEGs.’ Then I can drop a PDF onto icon, and the results appear with in folder I called, ‘Multi Page PDFs.’