SmugMug has 8 display sizes pre-made and ready to share.  Just use the share button to grab the link.
(See this help page for step-by-step instructions.)

If you were to embed each display size on a forum, this is what the code and and resulting images might look like.
You can also do custom image sizes, if you prefer.

VBulletin Forum code examples:

[img]http://www.smugmug.com/photos/257858359_ArAZw-Ti.jpg\[/img\]

[img]http://www.smugmug.com/photos/257858359_ArAZw-Th.jpg\[/img\]

[img]http://www.smugmug.com/photos/257858359_ArAZw-S.jpg\[/img\]

[img]http://www.smugmug.com/photos/257858359_ArAZw-M.jpg\[/img\]

[img]http://www.smugmug.com/photos/257858359_ArAZw-L.jpg\[/img\]

[img]http://www.smugmug.com/photos/257858359_ArAZw-XL.jpg\[/img\]

[img]http://www.smugmug.com/photos/257858359_ArAZw-X2.jpg\[/img\]

[img]http://www.smugmug.com/photos/257858359_ArAZw-X3.jpg\[/img\]