The basic idea:
Uploading your photo file onto the forum's server is a bit painful and (I believe) has a size restriction, so a much better option is to use a photo sharing site and use that to embed photos onto your post. This also allows you to use the same photo again on any post in any forum. I have always used PhotoBucket, and lots of other people like to use ImageShack, but the trendy thing to use seems to be Flickr. Since I don't have a Flickr account, I'm using this tutorial as an opportunity to create my own while documenting the process.
Creating a Flickr account
Going to the Flickr homepage and clicking on "Sign up now" gives us this new window:

This gives us several options for creating an account, depending on other services you may already be signed up for. If you already have a Facebook or Google account, you may find it easier to sign up through one of those services. If you have a Yahoo account you can obviously sign in with that. For the sake of the tutorial though, I'm going to click on "Create New Account".
This gives us this Yahoo signup form:

This should be fairly self-explanatory. Fill in your details, set a password and some security questions, type the text in the box and hit "Create My Account". You will already have an email address of some description in order to create your account on this forum, so I would recommend entering that into the "Alternative email" field.
So if all has gone to plan, hopefully you'll end up with a page that looks something like this:

Obviously I have blotted out my own personal details. As a general rule I would uncheck the box circled in red. If you entered in an alternative email address, you'll also get an email to that address asking to confirm the use of that email. It will take you to a page where you enter your Yahoo password and then your alternative email address is confirmed. Now let's go back to our confirmation page, click on the "Continue" button and see what happens...

This is fairly self-explanatory. Pick a screen name. And then...

Ta-da! Woah, lots of things to look at. But let's keep on track. We're here to upload some photos, so let's do that.
Uploading photos to Flickr
I just clicked on "Upload your first photos" (the one with the big red arrow crudely drawn on by yours truly.


Since you're going to be posting the photo on a public forum, I would choose to keep it at the default "public" setting.

The next step is simply to wait for it to upload. This could take a while, depending on the size of your photo and the speed of your Internet connection. Go grab a drink if you like. Note that this step can include uploading more than one photo at a time.

Hooray! Add a description? Sure, why not...

Here you can add some information about your photo if you like. You can see that I changed the title of mine. Then just hit "SAVE".

Hooray, your photo is on the Internet! Now all that's left is to find the code with which to share it on the forum. Click on the photo itself to continue.
Getting the code from Flickr to post on the forum
Once you're on the photo's page, you want to click on "Share" just above the photo, then "Grab the HTML/BBCode", then select "BBCode", as shown below.

Select all of the text in the box; right-click -> "Copy"; go to your post on the forum; right-click -> "Paste" and you should get something like this:
Code: Select all
[url=http://www.flickr.com/photos/66633832@N08/6058885229/][img]http://farm7.static.flickr.com/6072/6058885229_1ed1f2781d.jpg[/img][/url]
[url=http://www.flickr.com/photos/66633832@N08/6058885229/]A GT and a GT-R[/url] by [url=http://www.flickr.com/people/66633832@N08/]Paul F photos[/url], on Flickr
What that will give you is something like this:

A GT and a GT-R by Paul F photos, on Flickr
You can see that we have posted our image! Success!! It also has links to the photo on Flickr so that people can see it in different sizes and comment on it, and it also has a link to your own profile. You can remove this links if you like and just post what is in between the IMG and /IMG tags. For instance, in the above example, if you only wanted to post the image with no links you would delete all of the code except for this:
Code: Select all
[img]http://farm7.static.flickr.com/6072/6058885229_1ed1f2781d.jpg[/img]
Well, hopefully that just about covers it. If there's anything else, post a question in this thread and I'm sure I or someone else on the forum will be able to help you. I just really hope this helps newcomers to the forum and those who are trying to post "For Sale" threads etc in the future.