Images on the Web can be stored and transferred in a number of formats. All of these rely on the notion of a pixel - a picture element. Think of a pixel as one dot on your screen.
It is usually simple to convert from one image format to another. It is frequently necessary. Many programs do not save results in image formats, instead using postscript or another printer language. In that case we must convert to a reasonable compressed graphic format in order to display such information on the web. In this class, we will usually use the gimp to do that.
The gimp is the Gnu Image Manipulation Program. It is an open-software equivalent of Adobe Photoshop. To start it you may type "gimp" on a linux command line, or you may click Start->Graphics->The Gimp on a linux machine. Open an image by clicking the File -> Open menu. To save it in a different format, right-click the image, and select File->Save As. At the bottom of the dialog that results, type the name of the file in which you want to save the image, with the extension of the format you want, as seen above. Then just follow instructions. You may convert from any of the formats above to any other in this way.
As an alternative on Linux machines, one can use the convert command line utility. For example, to convert an existing JPEG image called myimage.jpg to PNG format, simply type the command
convert myimage.jpg myimage.pngon the Linux command line. This command invokes ghostscript and other utility programs to perform the conversion. This command has various options - see the man page for details.