It's really quite simple, and anyone can put a picture in their posts with just a little bit of basic knowledge. If you're a Premier Member you just happen to have a second, easier option.
The Basics
First it's important to understand how pictures work on the internet. In order for your picture to be seen by everyone, it must be "hosted" on a physical computer that is always connected to the internet, and has a reachable URL (universal resource locator). A URL is the thing that starts, "http://..."
One error a lot of members make is linking to a file on their computer. Entering something like, "C:\My Documents\..." will allow you to see your picture, but nobody else will because the image doesn't have an appropriate URL.
Almost all ISPs provide you with some webspace along with your account: a few megs of space where you can upload a webpage and pictures. It usually looks something like, "http://www.yourisp.com/~yourusername". If you have webspace, then you should use it to host your pictures (I'll explain how to put them in messages later on).
If your ISP doesn't provide you with webspace, there are a number of services that will host images for you -- some for free and some for a small fee. I've taken the time to look up a few of them for you, but I in no way endorse any of these, nor have I personally used their services.(NOTE: Brickshelf's focus is MOCs, or original Lego creations. Please refreain from using Brickshelf for other images, such as emoticons and avatars. Bottom line: Brickshelf is for Lego images only.)
Remember that you must use a hosting service -- a free web service like geocities.com and angelfire.com blocks your images from being viewed outside of their domain.
Getting Your Pictures Online
Now you need to get your pictures online. First, of course, you should take them into some image manipulation program (even just Microsoft Paint, which is included with Windows) and crop off any area that's not necessary. It's also a good idea to resize the image to less than 600 pixels wide to ensure that everyone can view it without having to scroll their browser window back & forth. This last part isn't mandatory, but it's good internet courtesy.
Now, depending on how you're getting your images hosted, you'll need to upload those files to the hosting computer. Some services provide web-based image uploading, while others will require that you use an FTP (file transfer protocol) program. Consult your hosting service or ISP for instructions.
Putting the Picture in Your Post
Standard Method
This is actually the method that I prefer because it allows me to put images of any size, and multiple images, in a single post.
Once your image is hosted outside of BZP, find the URL to the picture. For example, if you're using the webspace that your ISP provided, and uploaded a picture called "picture.jpg", the URL might be something like, "http://www.yourisp.com/~yourusername/picture.jpg".
To see if it's correct, type the URL directly into the "Address" window of your web browser. If your picture displays, then it's right. If anything other than your picture comes up, then it's back to the drawing board.
Assuming your URL is correct, you're almost home free. While posting or replying to a topic, click on the IMG button above the text-entry window and enter the URL for your picture.
This will insert the following code:
CODE
[img]http://www.yourisp.com/~yourusername/picture.jpg[/img]
Now when you post your message, that little string of code will be replaced with your picture! Ta-daaa!
Premier Member Method
As a courtesy to Premier Members who have contributed to the well-being of BZP, we offer this second option as a shortcut to posting images.
If your image is under 500 kilobytes (kB), you can simply attach the image to your post or reply by clicking the "Browse" button at the bottom of the message and locating the picture on your hard drive.
When you post the message, BZP will retrieve the image from your computer and host it for you right here on BZP's server -- no need to find external hosting.
The key limitation here is the filesize, and the restriction to one image per post.
Adding a Link to an Image
Sometimes you might want to link your image to a larger version of the image or to a page on BZPower or another website entirely. This can easily be done with the following code:
CODE
[url="Web Page you want linked to here"][IMG]Image URL here[/IMG][/url]
Additional Help
If there's something that is unclear or that you're unsure of, you can ask it in the official Pictures Topic.