How to use an image instead of an icon in flutter?

ImageIcon widget is the most suitable widget to use images as icons in Flutter. You can use its property image to assign your own image.

ImageIcon(
     AssetImage("images/icon.png"),
     color: Colors.red,
     size: 24,
),

Most of the time you'll find that the underling icon property is a Widget, so you can directly replace your Icon with Image.asset and provide it a width, height and fit according to your needs:

Image.asset(
  'assets/images/foo.png',
  width: 44,
  height: 44,
  fit: BoxFit.cover,
)

Example (in a TabBar)

TabBar(
  tabs: [
    Tab(
      text: 'Icon',
      icon: Icon(Icons.call), //    <-- Icon
    ),
    Tab(
      text: 'Image',
      icon: Image.asset( //        <-- Image
        'assets/images/foo.png',
        height: 44,
        fit: BoxFit.cover,
      ),
    ),
  ],
)

So follow up to @RobinSinha answer, using the Tab widget looks weirds as the Tab widget has an external padding, so i'd suggest to avoid that:

BottomNavigationBarItem(
       icon: Image.asset(
           "assets/images/image.png",
            width: <put your desired width, recommended 24.0>,
            height: <put your desired width, recommended 24.0>,
           ),
        label: '<name for icon>'
),

Little known fact. We can use PNG files in Flutter as an icon and change their color with color parameter like below:

 IconButton(
                        icon: ImageIcon(
                          AssetImage('assets/images/iconRandom.png'),
                          size: 30,
                          color: Colors.red,
                        ),
                        onPressed: () {
                          // do something here
                        }),

So no need to create several different image files just for changing colors depending on condition. We can just change color of the PNG file.

As @RobinSinha has said you can use Tab and then in for icon you can try the following

Tab(
        icon: Container(
          child: Image(
            image: AssetImage(
              'assets/logo.png',
            ),
            fit: BoxFit.cover,
          ),
          height: 100,
          width: 100,
        ),
      )