Well, it doesn't seem like my question was clear enough and I created my own solution.
Assuming the center of the circle is at x,y and the radius is r we can create a square with the points x-r, x+r, y-r, and y+r. This gives us a square that contains the circle. Then we can go through every pixel inside that square and determine if it's inside the circle using Pythagoras's equation. Basically it foes something like
int Left = (int)centerOfExplosion.Value.X - bombRadius;
int Right = Left + bombRadius * 2;
int Top = (int)centerOfExplosion.Value.Y - bombRadius;
int Bottom = Top + bombRadius * 2;
for (int j = Top; j <= Bottom; ++j)
{
for (int k = Left; k <= Right; ++k)
{
double dist = Math.Pow(centerOfExplosion.Value.X - k, 2.0) + Math.Pow(centerOfExplosion.Value.Y - j, 2.0);
if (dist <= Math.Pow(bombRadius, 2))
{
Color c = MapColorData[k + j * Map01Color.Width];
c.A = 0;
MapCollisionData[k + j * Map01Collision.Width] = Color.White;
MapColorData[k + j * Map01Color.Width] = c;
}
}
}