You can make one color in each image transparent.
Making a color transparent is helpful if you want to see information that is displayed behind the image. In a bitonal image, the transparent color is always the background color of the image. For grayscale and color images, you can choose the transparent color.
The transparent color is also used to replace data that is removed when you edit an image using the rub or crop commands. If background transparency is turned on for the image, the rubbed areas (or the slack space in a cropped image) become transparent. For more information, see Cropping Images and Removing (Rubbing) Images.
If you merge two images, the transparent color that is used to fill the space between the two images depends upon the display order you specify in the
Image Manager Toolspace. To use the transparent color of the first image in the list for the fill color, on the ribbon, click
Raster Tools tab 
Edit panel (expanded)
Merge Images or enter
iimerge at the Command prompt.
Sometimes you may want to designate an unused color as the transparent color. For instance, when merging images, turning on background transparency may make some desirable pixels transparent. The rbackground command allows you to designate an unused color in a grayscale image as the transparent color. Pixels with a value of 255 (white) are changed to a value of 254 (near white), then the transparent color is set to a value of 255. The resulting image will have no pixels with a value of 255.