From Apple Help:
Grab saves screen shots as files in TIFF format. If you want to use your screen shots on the web, in email, or in a word processor, you can use the Preview application to convert the TIFF files to other formats, such as JPEG or PNG.
To convert a Grab screen shot to another format:
Open Preview, located in Applications.
Choose File > Open and choose the file to convert.
Choose File > Save As and choose a file format from the Format pop-up menu.