Basic syntax for the img tag

The img tag is the simplest and most common way to display an image in an html page. It requires a src attribute, which indicates where the browser will find the image, and an alt attribute, which provides a textual version of the image for screen-readers.

An img element is, by default, an inline element, so it will wrap with other inline elements on the page.

Sizing an img

img tags can be sized with CSS. Like other elements, you can add an id, class, or other attributes as you see fit in order to assist with styling. Generally, it is best to set only width or height, not both. If you set only one, the browser will set the other automatically in order to maintain the aspect-ratio of your image source file.

Cropping an img

Sometimes your source image doesn't match the aspect-ratio of the space you need to fill. What if we want to put our wide image into a square? First, let's try just setting both width and height.

The image is stretched! Not what we wanted. We need to tell the image to be cropped, not stretched.

Now the image is cropped, instead of stretched. The cover value tells the browser to scale the image proportionally such that it just covers the entire area of the img tag, but no larger. (You can also use the contain value, which scales the image proportionally to fit entirely inside the area of the imgtag.) The image will be centered by default, but the center is not really the best part of this particular image, so let's reposition it.

The object-position value can have two parts, the first for the horizontal position, the second for vertical position. We can use words like left, right, and top, or numerical values like 100px or 20%.

Background image syntax

The img tag is useful when the image is part of your actual content, but it's often not ideal when using an image as a style element behind other content. CSS provides a mechanism for using images as backgrounds to other elements. Let's add a background image to the example definition below.

You'll see that the background image only extends as far as the edges of the element being styled — so in this case, only a few sentences. Let's add some padding to that div, so we can see more of the image.

Sizing and positioning background images

We can use background-size: cover; to scale the background image to fit its container, similar to object-fit: cover; for img tags. Similarly, we can use background-position to control the position of the image within the frame. The background-position value can have two parts, a horizontal and vertical component, just like object-fit did. In this case, center 20% works nicely.

Selecting the right file type

A number of factors come into play when selecting which file type to use for a given image. Is the image raster (like a photo, comprised of a grid of pixels), or vector (described by mathematical equations, like an illustrator file)? Is it going to be displayed at large sizes and therefore must be well-compressed to minimize download time, or will it be very small, such that compression won't matter as much? Does it require transparency? Must it animate? The following table compares the four most commonly-used image file types.

jpg	gif	png	svg
36KB	138KB	422KB	Not applicable, SVG is vector only!
Pros: Very small file size for photos	Pros: Can be animated Small file size for solid colors	Pros: Lossless compression Full transparency Works for photos and graphics	Pros: Lossless compression Full transparency Infinite scaling Usually small file size
Cons: Lossy compression Adds artifacts in areas of high contrast No transparency	Cons: limited color palette High noise/banding Larger file size 1-bit transparency (rough edges)	Cons: Very large file size	Cons: No support for raster images

Managing file size (and therefore download time) for raster images

Image fidelity and file-size are inextricably linked. Both are a function of the number of pixels in the image, and the amount/type of compression applied. These are both variables over which you have control, and which you must control in order to find the right balance between quality and download-time.

First, decide what is the maximum width at which this image is displayed? For instance, in this document, the main column is constrained at 800px wide, and will never get wider. An image wider than 800 pixels is therefore unnecessary, we use crop and/or image sizing tools in Photoshop to make the image be 800px wide.

Second (at least for a jpg image), we decide how much compression to apply. This is best done using Photoshop's 'Export As...' feature, which opens the image in a window that allows you to try out compression settings while seeing their effet on image quality. In most cases, jpg compression higher than 60% is unnecessary, and 35-45% is often fine.

Taking these two steps when preparing an image for use can make a dramatic difference in file size, and therefore download time. It's not uncommon to reduce the file to a 10^th or even 100^th of it's original uncompressed size, meaning it will download more quickly — by orders of magnitude.