First, select your image to brighten. Next, click the "Color Tuning" tool icon. You'll want to then click the "Highlight/Shadows" tab, as seen below.

Here's where it gets real subjective; trial and error, skill, luck - whatever - plays an active part here. As the second sentence above states, dragging the little squares up or left makes the sprite brighter. The trick is, you need to drag all 3, and balance them out, to get acceptable results. Huh? Never mind. Take a look at the squares below to see what I mean.

Notice the line is now a curve. In this example, I moved all 3. Most of the time you'll need to do the same, sometimes one more than the others. It all depends on what you have to work with. Click Apply (duh!) to see the effect on your sprite. ;-)

You'll likely want to make several attempts. MIC makes this easy with the Undo icon (or press Ctrl + Z). Just make sure to do this immediately after clicking Apply. Keep modifying the curve until you get the results you want.