Sure, here are the steps on how to resize an image based on the longest side:

1. Open the image in an image editing software.
2. Find the width and height of the image.
3. Calculate the longest side by comparing the width and height.
4. Set the width or height of the image to the longest side.
5. Save the image.

Here is an example of how to resize an image based on the longest side using Photoshop:

1. Open the image in Photoshop.
2. Go to the Image menu and select Image Size.
3. In the Image Size dialog box, set the Width or Height to the longest side of the image.
4. Leave the Constrain Proportions checkbox checked.
5. Click on the OK button.

The image will be resized based on the longest side.

Here is an example of the code you can use to resize an image based on the longest side in Python:

Python

import PIL.Image

def resize_image(image, longest_side):
  width, height = image.size
  if width > height:
    new_width = longest_side
    new_height = int(new_width * height / width)
  else:
    new_height = longest_side
    new_width = int(new_height * width / height)

  image = image.resize((new_width, new_height))
  return image

image = PIL.Image.open("image.jpg")
resized_image = resize_image(image, 100)
resized_image.save("resized_image.jpg")

This code will resize the image to 100 pixels on the longest side.