How to Change the Format in Apple iPhoto
Many Mac owners use iPhoto, included with every new Mac, to organize their photos. If you use iPhoto and have not browsed through the menus, however, you may be unaware of some of the advanced features of the program. For example, iPhoto has the ability to take any image in your photo library and export it in a completely different format. This allows greater flexibility for manipulating images in outside programs and can allow you to make images easier to view on older operating systems. Use iPhoto to change an image from one format to another.
Locate the image in your iPhoto library that you would like to change the format of, and click it.
Click the "Share" menu at the top of the window, then click "Export."
Confirm that the "File Export" tab is selected, then use the "Format" drop-down menu to select the new format for the image. The available choices include JPG, TIFF and PNG.
Use the radio buttons under "Size" to select whether the image should be converted using its original size, or a different size. If "Scale images" is selected, enter the new image size (in pixels) using the boxes below.
Use the radio buttons under "Name" to select how the exported file should be named. It can be named according to the name of the original file, or the name that you gave it when importing it into the iPhoto library.
Click the "Export" button. Select the location on your computer where the exported image from iPhoto should be saved, then click "OK" to export the photo.