DSE ICT — Multimedia and Web Authoring | Tutor Prep Guide (for tutors)

2026-11-02

Tutor lesson-prep guides · Secondary ICT

A 1200 by 800 image at 24 bits per pixel is 2.75 MiB uncompressed. Students say “it is big so it is slow” but cannot compute how big. This lesson makes them compute it.

Objectives — Students can: (1) compute the size of a 1200 by 800 image at 24 bits (2.75 MiB) and its size after 12:1 compression (about 234 KiB); (2) choose between bitmap and vector formats for a given use with two reasons; (3) write alt text and heading levels for a page with 1 images that meets basic accessibility rules.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Unit conversion	How many bytes are in one KiB	1,024 bytes
Binary	How many colours can 8 bits represent	256, that is two to the power of eight
Basic HTML	Which tag and attributes display an image	img with src and alt

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Unit warm-up	Count from 1 KiB up to 1 MiB	Three conversions correct
8-22 min	File size calculation	Model width times height times depth divided by eight	Get 2.75 MiB
22-34 min	Format trade-offs	Compare three formats on one image	Two reasons given
34-46 min	Web accessibility	Model alt text and heading levels	Every image has alt
46-54 min	Timed practice	Four calculations and one explanation	All finished
54-60 min	Exit check	Three quick questions	All three correct before leaving

03 Teaching script (read it out)

Scene 1: File size is calculated, not guessed

Formula: width times height times bit depth divided by eight, in bytes.
1200 times 800 times 24 divided by eight is 2,880,000 bytes, that is 2.75 MiB.
Students must press the calculator themselves; copying the answer collapses as soon as the numbers change.

Ask the student: what happens to the file size when the depth goes from 24 to 32 bits.

Scene 2: Compression trades quality for space, and the cost must be named

12:1 compression leaves about 234 KiB, a saving of 92%; the cost is blurred detail, especially around text edges.
A 500 KiB page budget with 60% for images leaves 300 KiB, enough for about 1 images of this size.
A vector graphic with 40 nodes is about 1280 bytes and scales without loss, but cannot represent a photograph.

Ask the student: which format for a logo and which for a photograph, and why.

Scene 3: Accessibility is a baseline, not a bonus

Every meaningful image needs alt text describing its content or function, not the word image; decorative images take an empty alt.
Heading levels run in order: one h1, then h2 and h3 without skipping.
Contrast and text size matter as much as alt text.

Ask the student: what alt text an image used as a button should carry.

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Image file size	Width times height times depth divided by eight, then convert units	2.75 MiB
Calculation	Compressed size	Original size divided by the ratio	234 KiB
Explanation	Format trade-off	Cover use, quality and file size	One sentence each
Practical	Marking up an image	img with src and alt, plus heading levels	Specific non-empty alt

Sanity check (teach this): Reverse-check every answer: multiply by eight and divide back by width, height and depth to recover the original numbers; always state KiB or MiB, an answer without units is wrong.

05 Independent practice (3 questions)

Question	Answer	What to watch
Compute the size of a 1200 by 800 image at 24 bits	Write the formula line by line	2.75 MiB
Compute the same image at 12:1	Divide the raw size by the ratio	234 KiB
Write alt text for three images	Describe content and function	All three specific
Explain why a logo should be a vector	No loss when scaled, small file	Two reasons

06 Common mistakes and what to say

Mistake	Student writes	Say this
Forgetting to divide by eight	Bits treated as bytes, answer eight times too large	“Bit depth is in bits, not bytes.”
Mixing KiB and KB	Answer off by a whole order of magnitude	“1,024 and 1,000 are not interchangeable.”
Saying “big so it is slow”	No marks on an explanation question	“How big? Give me the number.”
Writing alt as image	All accessibility marks lost	“alt describes the content, not that it is a picture.”

07 Homework (paper, section and question numbers)

Three file size calculations — Practice paper questions 1 to 3
One format trade-off explanation — Practice paper question 5
Alt text and headings for a four-image page — Practice paper question 6

08 Exit check (last 5 minutes)

Question	Answer	Decision
What is the formula for image size?	Width times height times depth divided by eight	Do not forget the division
How large is 1200 by 800 at 24 bits?	2.75 MiB	About 2812 KiB
What should alt text say?	Content or function	Never the word image

09 Tutor FAQ

Students keep dropping the divide by eight, what works?

Require the formula in writing and a reverse check every time.

Do they need to memorise every format?

Three is enough: JPEG for photos, PNG for transparency, SVG for icons.

Is accessibility examinable?

Yes, alt text and heading levels appear as explanation questions.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.