We've all been there: you forgot to scale down an image before adding it to the page. The image looks fine, but it is wasting users' data and hurting page performance.
Identify incorrectly sized images
Lighthouse makes it easy to identify incorrectly-sized images. Run the Performance Audit (Lighthouse > Options > Performance) and look for the results of the Properly size images audit. The audit lists any images that need to be resized.
Determine the correct image size
Image sizing can be deceptively complicated. For this reason, we've provided two approaches: the "good" and the "better." Both will improve performance, but the "better" approach may take a bit longer to understand and implement. However, it will also reward you with bigger performance improvements. The best choice for you is the one that you feel comfortable implementing.
A quick note on CSS units
There are two types of CSS units for specifying the size of HTML elements, including images:
- Absolute units: Elements styled using absolute units will always be displayed at the same size, regardless of device. Examples of valid, absolute CSS units: px, cm, mm, in.
- Relative units: Elements styled using relative units will be displayed at varying sizes, depending on the relative length specified. Examples of valid, relative CSS units: %, vw (1vw = 1% of the width of the viewport), em (1.5 em = 1.5 times font size).
The "Good" Approach
For images with sizing based on…
- Relative units: Resize the image to a size that will work across all devices.
You may find it helpful to check your analytics data (e.g. Google Analytics) to see which display sizes are commonly used by your users. Alternatively, screensiz.es provides information about the displays of many common devices.
- Absolute units: Resize the image to match the size that it is displayed at.
The DevTools Elements panel can be used to determine what size an image is displayed at.
The "Better" approach
For images with sizing based on…
- Absolute units: Use srcset and sizes attributes to serve different images to different display densities. (Read the guide on Responsive Images here.)
"Display density" refers to the fact that different displays have different densities of pixels. All other things being equal, a high pixel density display will look sharper than a low pixel density display.
As a result, multiple image versions are necessary if you want users to experience the crispest possible images, regardless of the pixel density of their device.
Some sites find that this difference in image quality matters, some find that it does not.
Responsive image techniques make this possible by allowing you to list multiple image versions and for the device to choose the image that works best for it.
- Relative units: Use responsive images to serve different images to display sizes. (Read the guide here.)
An image that works across all devices will be unnecessarily large for smaller devices. Responsive image techniques, specifically srcset and sizes, allow you to specify multiple image versions and for the device to choose the size that works best for it.
Regardless of the approach that you choose, you may find it helpful to use ImageMagick to resize your images. ImageMagick is the most popular command line tool for creating and editing images. Most people can resize images far more quickly when using the CLI than a GUI-based image editor.
Resize image to 25% the size of the original:
convert flower.jpg -resize 25% flower_small.jpg
Scale image to fit within "200px wide by 100px tall":
convert flower.jpg -resize 200x100 flower_small.jpg
magick convert flower.jpg -resize 200x100 flower_small.jpg
If you'll be resizing many images, you may find it more convenient to use a script or service to automate the process. You can learn more about this in the Responsive Images guide.
Once you've resized all your images, re-run Lighthouse to verify that you didn't miss anything.