How can I rename multiple pictures simultaneously?
Using the built-in tool "Automator", you can define a workflow to batch rename files.

To do that, you will have to:


Open Automator find it using Spotlight or in /Applications
Create a new Workflow
Type get into the search field 
Scroll to the bottom of the list of items and drag Get Specified Item from the Files &amp; Folders actions list on the left panel and drop it into the right panel
Click the Add button and find your files
Add the Sort Finder Items action followed by Rename Finder Items
It the "Rename Finder Items" panel, select "Make Sequential" and "Add number to" Picture
Hit the play button and you should be done


Have a look here or here for an example tutorials on your specific renaming task at hand. An excellent overview of what Automator does and how powerful (and simple) it can be as a personal assistant - see the web page http://macosxautomation.com/automator/index.html