Image resizing algorithm

I want to write a function to downsize an image to fit specified bounds. For example i want to resize a 2000x2333 image to fit into 1280x800. The aspect ratio must be maintained. I've come up with the following algorithm:

NSSize mysize = [self pixelSize]; // just to get the size of the original image
int neww, newh = 0;
float thumbratio = width / height; // width and height are maximum thumbnail's bounds
float imgratio = mysize.width / mysize.height;

if (imgratio > thumbratio)
{
    float scale = mysize.width / width;
    newh = round(mysize.height / scale);
    neww = width;
}
else
{
    float scale = mysize.height / height;
    neww = round(mysize.width / scale);
    newh = height;
}

And it seemed to work. Well ... seemed. But then i tried to resize a 1280x1024 image to a 1280x800 bounds and it gave me a result of 1280x1024 (which obviously doesn't fit in 1280x800).

Does anybody have any ideas how this algorithm should work?