In our last blog posting on this topic, I wrote about some free software you can use to edit your images for web. In this article, I want to focus more on the practicalities of image size - why it is so important that your images are optimised, and what the real difference in image size means.
A couple of important rules of thumb for you to remember:
1. Image size - between 4kb and 20kb per image (depending on the no of images per page)
2. Page size - each page on your site should be a maximum of 50-80kb in total (though I reckon in these days of broadband that going to 150-200kb is probably acceptable).
3. Stickability - 50% of people leave the average website within 30 seconds! If your website hasn’t even loaded after 30 seconds, how many people will hang around?
I have recently had some clients who have experienced some challenges from this issue of image size - both clients I am thinking of have shopping cart (or ecommerce) websites. One of them had product images that were about 50kb and the other had product images in the order of 400kb!
Now, just having one image that is that much bigger than standard (4k to 20k in size) is not going to be a huge issue. But in shopping carts, you might have a page with 30 of these images showing at a time! 30 images at 400kb each is a whopping 12,000kb (12 MB) per page! Even at 40kb per image you are still looking at 1200kb (1.2MB) per page.
How long does it take to fully load a web page that is 1.2Mb or 12Mb in size? Lets check the
Page Size - 50kb
7 secs - Dial Up - At 56kb (which is quicker than people on Dial Up usually get)
1 sec - Very Slow Broadband (256KB)
Instantly - Slow Broadband (512kb)
Instantly - Medium Broadband (1500kb)
OK Page Size (150kb)
21 secs - Dial Up - At 56kb (which is quicker than people on Dial Up usually get)
4 secs - Very Slow Broadband (256KB)
2 secs - Slow Broadband (512kb)
Instantly - Medium Broadband (1500kb)
Huge Page Size (1200kb)
2 mins 55 secs - Dial Up - At 56kb (which is slightly quicker than actual Dial Up speed)
36 secs - Very Slow Broadband (256KB)
18 secs - Slow Broadband (512kb)
9 secs - Medium Broadband (1500kb)
Massive Page Size (12,000kb)
29 mins 15 secs - Dial Up - At 56kb (which is slightly quicker than actual Dial Up speed)
6 mins - Very Slow Broadband (256KB)
3 mins - Slow Broadband (512kb)
1 min - Medium Broadband (1500kb)
So - you can see the problem with large image files!
From a speed perspective there is no question that you want small file sizes. So why would anyone want to use large image files? To preserve image size (ie the physical dimensions of the image) and image quality (low-resolution, medium-resolution, high resolution, very high resolution). If you are selling products (or services for that matter), you want the customer to be able to see the product in its best light - the image needs to be clear (not blurry) and large enough to see all the detail.
So, it comes down to a choice between SPEED & CLARITY!
The great thing about the way images display on the web, is that usually you can look at a fairly low resolution image and it looks pretty similar to a much higher resolution image.
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