Serve images with correct dimensions

Katie Hempenius
Katie Hempenius

Run Lighthouse

This Glitch is small enough that its images could be inspected by hand. However for most websites, using a tool like Lighthouse to automate this is essential.

  1. To preview the site, press View App. Then press Fullscreen fullscreen.
  2. Press `Control+Shift+J` (or `Command+Option+J` on Mac) to open DevTools.
  3. Click the Lighthouse tab.
  4. Make sure the Performance checkbox is selected in the Categories list.
  5. Click the Generate report button.
  6. Look for the results of the Properly Size Images audit.

The properly size images audit failing in Lighthouse.

The Lighthouse audit shows that both of this page's images need to be resized.

Fix flower_logo.png

Start at the top of the page and fix the logo image.

  • Inspect flower_logo.png in the DevTools Elements panel.

The DevTools elements panel

This is the CSS for flower_logo.png:

.logo {
  width: 50px;
  height: 50px;

The CSS width of this image is 50 pixels, so flower_logo.png should be resized to match. You can use ImageMagick to resize the image to fit. ImageMagick is a CLI tool for image editing that comes pre-installed in the codelab environment.

  1. Click Remix to Edit to make the project editable.
  2. Click Terminal (note: if the Terminal button does not show you may need to use the Fullscreen option).
  3. In the console, type:
convert flower_logo.png -resize 50x50 flower_logo.png

Fix flower_photo.jpg

Next, fix the photo of the purple flowers.

  • Inspect flower_photo.jpg in the DevTools elements panel.

The DevTools elements panel

This is the CSS for flower_photo.jpg:

.photo {
  width: 50vw;
  margin: 30px auto;
  border: 1px solid black;

50vw sets the CSS width of flower_photo.jpg to "half the width of the browser." (1vw is equal to 1% the width of the browser).

The ideal size for this image would depend on the device it is being viewed on, so you should resize it to a size that works well for most of your users. You can check your analytics data to learn which screen resolutions are common amongst your users:

Google analytics of screen resolutions.

This data indicates that 95%+ of the visitors to this site use screen resolutions 1920 pixels wide or less.

Using this information we can calculate how wide the image should be: (1920 pixels wide) * (50% of browser width) = 960 pixels

On resolutions greater than 1920 pixels wide, the image will be stretched to cover the area. The resized image is still fairly large, so the effects of this should not be very noticeable.

  • Use ImageMagick to resize the image to 960 pixels wide. In the terminal type:
# macOS/Linux
convert flower_photo.jpg -resize 960x flower_photo.jpg

# Windows
magick convert flower_photo.jpg -resize 960x flower_photo.jpg

Re-Run Lighthouse

  • Re-run the Lighthouse Performance audit to verify that you have successfully re-sized the images.

Lighthouse properly size images audit.

… And it fails! Why is that?

Lighthouse runs its tests on a Nexus 5x. The Nexus 5x has a 1080 x 1920 screen. For the Nexus 5x, the optimal size of flower_photo.jpg would be 540 pixels wide (1080 pixels * . 5). This is much smaller than our resized image.

Should you resize the image to be even smaller? Probably. However, the answer to this isn't always clear-cut.

The trade-off here is between image quality on high-resolution devices and performance. It's easy to overestimate how closely users will be inspecting images—so you should probably make them smaller—but there are certainly use cases where image quality is more important.

The good news is that you can bypass this tradeoff altogether by using responsive images to serve multiple images sizes. You can learn more about this in the Responsive Images guide.