Black and White
There is a classic feel to black and white pictures. If your bathroom is an older style that features a lot of white ceramics, then black and white may work well in that room. Black and white prints take away the worry of matching the different shades of color when you are accessorizing your bathroom.
Framed
An advantage in framing pictures is you can use the same picture and just change the frame whenever you redecorate your bathroom. The disadvantage to using frames is the additional cost in buying them. If you are using frames, make sure the pictures are first mounted. If you simply place the picture inside the frame it will wrinkle from humidity.
Dry Mounted
Dry mounted pictures are a good choice for humid environments. Regardless of their size, once a photo is dry mounted it will not wrinkle if exposed to humidity. Another advantage is in most cases a protective coating is used during the dry mount process, allowing easy cleaning of the mounted picture.
Your Own Photos
Another option is to decorate your bathroom with many of your own family photos. Buy a number of different frames of varying sizes and continually update the pictures inside the frames as your family grows up.