A function $f$ is defined on the complex numbers by $f(z)=(a+bi)z,$ where $a$ and $b$ are positive numbers.  This function has the property that the image of each point in the complex plane is equidistant from that point and the origin.  Given that $|a+bi|=8$ and that $b^2=m/n,$ where $m$ and $n$ are relatively prime positive integers.  Find $m+n.$
1. Given the function \( f(z) = (a + bi)z \), where \( a \) and \( b \) are positive numbers, and the property that the image of each point in the complex plane is equidistant from that point and the origin, we have:
   \[
   |f(z)| = |z - f(z)|
   \]
   This implies:
   \[
   |(a + bi)z| = |z - (a + bi)z|
   \]

2. Simplify the right-hand side:
   \[
   |(a + bi)z| = |z(1 - (a + bi))|
   \]
   Since \( |kz| = |k||z| \) for any complex number \( k \):
   \[
   |a + bi||z| = |1 - (a + bi)||z|
   \]
   Dividing both sides by \( |z| \) (assuming \( z \neq 0 \)):
   \[
   |a + bi| = |1 - (a + bi)|
   \]

3. Let \( a + bi = c \). Then:
   \[
   |c| = |1 - c|
   \]
   Given \( |c| = 8 \):
   \[
   8 = |1 - c|
   \]

4. Let \( c = a + bi \). Then:
   \[
   |1 - (a + bi)| = 8
   \]
   This can be written as:
   \[
   |(1 - a) - bi| = 8
   \]

5. The magnitude of a complex number \( x + yi \) is given by \( \sqrt{x^2 + y^2} \):
   \[
   \sqrt{(1 - a)^2 + b^2} = 8
   \]

6. Squaring both sides:
   \[
   (1 - a)^2 + b^2 = 64
   \]

7. We also know that:
   \[
   a^2 + b^2 = 64
   \]

8. We now have two equations:
   \[
   (1 - a)^2 + b^2 = 64
   \]
   \[
   a^2 + b^2 = 64
   \]

9. Subtract the second equation from the first:
   \[
   (1 - a)^2 + b^2 - (a^2 + b^2) = 64 - 64
   \]
   \[
   (1 - a)^2 - a^2 = 0
   \]
   \[
   1 - 2a + a^2 - a^2 = 0
   \]
   \[
   1 - 2a = 0
   \]
   \[
   a = \frac{1}{2}
   \]

10. Substitute \( a = \frac{1}{2} \) into \( a^2 + b^2 = 64 \):
    \[
    \left(\frac{1}{2}\right)^2 + b^2 = 64
    \]
    \[
    \frac{1}{4} + b^2 = 64
    \]
    \[
    b^2 = 64 - \frac{1}{4}
    \]
    \[
    b^2 = \frac{256}{4} - \frac{1}{4}
    \]
    \[
    b^2 = \frac{255}{4}
    \]

11. Given \( b^2 = \frac{m}{n} \) where \( m \) and \( n \) are relatively prime positive integers, we have \( m = 255 \) and \( n = 4 \). Therefore:
    \[
    m + n = 255 + 4 = 259
    \]

The final answer is \(\boxed{259}\)