TOPIC:

[img size=(01-499)]www.google.com/images/web_logo_left.gif[/img]

is the recipe.

in the examples below
[img size=40...
[img size=80...
[img size=160...
and finally
[img size=320...

Note lack of (parentheses)

The image gets resized as much as possible. If you always use 499 you'll get the biggest image.

If you want to link to an image you use a small image as the text part of the bbcode url recipe.