Basically, "transparent" is a color. Usually, it is called the "alpha, channel" and it can be set to be 0-100% opaque. Editing programs will allow any color to be replaced by the alpha channel.

Note that there are potential issues, depending on the image details. For example, if an image element fades into the background color and one replaces the background color with the alpha channel, the faded edge will probably look raggedy. In this case you'll need to blend the edges. The editing program may or may not be helpful for accomplishing this task.