Choosing an image format means matching the file type to what the image shows and where it will be used. Questions usually ask you to name a format and give a reason, so the reason must refer to the specific image.
All file names, sizes and documents in this lesson are invented for practice. Menu names differ between programs and versions, so the steps below describe the feature, not one brand’s buttons. Always confirm the formats named in the current syllabus on the Cambridge IGCSE ICT 0417 page.
What are the main formats good for?
| Format | Strength | Typical use |
|---|---|---|
| JPEG | Small files for photos using lossy compression | Photographs on web pages |
| PNG | Lossless, supports transparent areas | Logos, screenshots, diagrams |
| GIF | Few colours, simple animation | Small animations, simple icons |
| Vector (such as SVG) | Scales to any size without losing sharpness | Logos and line drawings |
Lossy compression discards some detail to save space. Lossless compression keeps every pixel exactly. A bitmap is a grid of pixels, while a vector image stores shapes as instructions.
How do you build the explanation?
- State what the image is: photo, logo, diagram or animation.
- State where it will be used: web page, print or email.
- Name the format.
- Link a property of the format to a need: file size, quality, transparency or scaling.
Worked example
A website needs a photograph of the school hall, shown at 800 × 600 pixels.
Step 1: an uncompressed 24-bit image uses 3 bytes per pixel. Pixels = 800 × 600 = 480,000.
Step 2: size = 480,000 × 3 = 1,440,000 bytes, about 1.44 MB, before compression.
Step 3: a photo has many colours and gradual changes, so JPEG compression greatly reduces the file at an acceptable quality loss.
Answer: JPEG, because the image is a photograph with many colours and the page loads faster with a smaller file.
The mistake to watch for
Mistaken answer: “PNG, because it is good.”
This names a format but gives no reason linked to the content. The correction is to say what the image is and which property helps: “PNG, because the logo has flat colours and needs a transparent background.”
Check yourself
1. Calculate the uncompressed size of a 1000 × 500 pixel image using 3 bytes per pixel.
Show answer
Pixels = 500,000. Size = 500,000 × 3 = 1,500,000 bytes.
2. An invented club logo with a transparent background must appear on a coloured web page. Suggest a format and give a reason.
Show answer
PNG, because it is lossless and supports transparent areas, so the coloured page shows through.
3. Give one reason to prefer a vector image for a logo that will be printed on both a badge and a banner.
Show answer
A vector image scales to any size without becoming blurry or pixelated, so one file serves both uses.
Where this leads next
Test everything in the module practice set. The module guide shows how this fits with the other lessons.
If your reasons sound right in your head but thin on paper, a teacher in online one-to-one ICT tuition can help you phrase them in marking-scheme style.