DX Image Dimensions

DX in common with many other forums have a guideline for the sizing of images posted

DX Guideline = Image dimensions must be no more than 640x480 pixels with a file size of no more than 55K,
keep in mind that even this much may take over 5 seconds to fully appear.

Here are some examples of why the guideline of 640x480 pixels was chosen
I have used this Cobalt1959 (aka Eddie) post to illustrate the problems (in no way am I implying that this post was faulty - in fact it was better than perfect regarding sizes)

You will need to imagine that the images below are as they would be viewed on your monitor
(all these images are 640x465 pixels and between 36 to 44k file size)

1600x 1280 pixels , absolutely useless for viewing without scrolling, left, right, up and down
View attachment 31511

1280 x 960 pixels , far too big for viewing, without scrolling
View attachment 31510

1024x768 pixels , still too big for viewing.
View attachment 31509

640 x 480 pixels , perfect for viewing on any monitor.
View attachment 31508

The cars shown are the Rover BRM (Sports Saloon) Rover BRM (LeMans Gas Turbine powered car) and a BRM Grand Prix car

Avoid putting off your viewers
Since the average web surfer will, only wait on average 5 seconds for your web page to download (or appear in their browser) it is important to have small file sizes on your web pages so, they download quickly. Most usability experts (those who study how people like to use the we), suggest files sizes of 25 KB - 50 KB. Although broadband (high speed Internet access with cable, DSL, and wireless), has made tremendous strides in market penetration, obtaining critical mass at 33% of Internet users, nevertheless nearly 67% of web users still have slow dial up connections, (these figures are ever changing - please correct me if I am wrong)

Images are used on web for many reasons: branding, highlighting, navigation, providing supplementary information, division, humour etc. Images consume more space than text, however, and thus take a longer time to download. If you feel you want to show a large image, display a thumbnail image instead, and link this to the larger image with a message such as 'Click to view the full-sized (200 KB) image.' Minimise the use of animated GIFs (images that endlessly cycle through a short animation.) Not only do these appear garish (and frequently annoying), but they are larger than still images and thus slower to appear.

Photographs will generally be smaller (and look better) if saved in JPG format. Cartoon-like images, will generally be smaller (and look better) if saved in GIF format. Most images will be more useful if accompanied by a caption. If you can't think of a relevant caption, this may be a sign that you don't need the image.
Armed with these tips, your images will load quickly and will be well regarded by your viewers.