Apple's Preview program in OS X is a very convenient tool for quickly editing image and PDF files. It supports a number of options for annotating images and adjusting colors, cropping, and otherwise editing them, it can also be used to quickly convert an image to another format.
Preview supports reading most common image format types, and likewise allows you to export them to these same types, so even if Preview is not set to be your default program for handling images, you can still use it to quickly make an image conversion.
To do this, right-click the image in the Finder and choose Preview from the Open With menu. Then choose either Export or Save As from the File menu, and in the Save dialog box you will see a Format drop-down menu that you can use to select the desired image format, which will be either JPEG, JPEG2000, OpenEXR, PDF, PNG, or TIFF. When you select a format, you may see some adjustment options available, such as enabling the alpha channel for PNGs to include transparencies, and setting the compression level for JPEG images.
These options are available for single images, but can be applied to batches of images as well. To do this, instead of opening a single image at a time, select a number of them in the Finder and likewise right-click to open the selection in Preview. When opening them in this manner, they will be gathered in a single Preview window that will list them as thumbnails to the left of the main viewing area. With this view, you can then hold the Command key and select some of the images in random order, use the Shift key to select a series of images in order, or select one and then press Command-A to select all of them.
With your desired images selected, the Export option in the File menu will change to Export Selected Images, which will give you the same options to save them in the desired format. Do keep in mind that when using batch conversions you will not be able to name the files differently, so they will keep their original names but have a new file type suffix associated with them.
It may be good practice to use separate folders to group the source and destination images, which may make it easier for managing large conversion sets.