A tiny ammendment to zorroswordsman at gmail dot com's resize class. It was only resizing if both the width AND height where greater than the desired size. This should fix it:

<?php
// Set maximum image size (pixels)
function set_size($size = 100)
{
    // Resize
    if($this->x_input > $size || $this->y_input > $size)
    {
        // Wide
        if($this->x_input >= $this->y_input)
        {
            $this->x_output = $size;
            $this->y_output = ($this->x_output / $this->x_input) * $this->y_input;
        }
        // Tall
        else
        {
            $this->y_output = $size;
            $this->x_output = ($this->y_output / $this->y_input) * $this->x_input;
        }
        // Ready
        $this->resize = TRUE;
    }
    // Don't resize
    else { $this->resize = FALSE; }
}
?>