update readme to include best practices for image data optimization

[bhimrazy]

What does this PR do?

This PR updates readme to include best practices for image data optimization.

def random_images(index):
+   # Replace with your actual image loading here (e.g., .jpg, .png, etc.)
+   # (recommended to pass as compressed formats like JPEG for better storage and optimized streaming speed)
+   # You can also apply resizing or reduce image quality to further increase streaming speed and save space.
    fake_images = Image.fromarray(np.random.randint(0, 256, (32, 32, 3), dtype=np.uint8))
    fake_labels = np.random.randint(10)

    # You can use any key:value pairs. Note that their types must not change between samples, and Python lists must
    # always contain the same number of elements with the same types.
    data = {"index": index, "image": fake_images, "class": fake_labels}

    return data

Partially resolves #573.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79%. Comparing base (1dadc00) to head (75f92bf).
Report is 2 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #577   +/-   ##
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  Coverage    79%    79%           
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  Files        40     40           
  Lines      6111   6111           
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  Hits       4812   4812           
  Misses     1299   1299           

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