In Linkedin, best image size is 698x400 pixels.Want to know the same for images appearing before an article in Joomla 3.0 (like Need a Writer... image, http://bit.ly/15ZvlcR).
There is no general rule for Joomla! It depends on the width of the position you are putting the image in. Which depends on the template used and the layout the site uses. For example the width of the position on your example seems to be 586px. But the position right of it (drm-free) is 266px.
Easiest way to find the correct sizes is to use Firebug or some other web development extension. They can tell the sizes for any elements directly.
Also, if you use a responsive layout (not in the example), the optimal size typically varies based on the widht of the browser window. Fortunately the image scaling algorithms the browsers use approach usability as long as the image is not too small by a wide margin. But if you want optimal image quality on a responsive design you'll have to go beyond this answer.
The answer depends on your requirements for the best image.
- It can the smallest image possible for a device
- You can deliver different images for each device
- You can show the biggest image possible to match social networks sharing requirements:
Right-sizing Images - Mixing Responsive and Adaptive Techniques https://www.youtube.com/watch?v=8WLoOFqC1NA&index=30&list=PLSwFVRVZ3joTo3InkU_FT65_AK8dQiyZ4
Social Media Profile Images Sizes http://www.extly.com/docs/autotweetng_joocial/faq-social-media.html
As other people have said, it will depend on your theme. The only guide with images is that they shouldn't be larger or smaller than the largest size they appear on the site, as stretching images causes pixellation and shrinking images means users are downloading more data than they need to.
If we look at LinkedIn specifically, users can upload any size images to their site, and so LinkedIn need to decide how to handle that.
They display images at 698x400px, and also at a smaller thumbnail size which keeps the same scale. If someone uploads a different scale image, they will then need to crop it programatically. From a user's perspective, a cropped image won't look as good. This is the reason why people recommend uploading to LinkedIn at 698x400px - what you upload will then be what you see.