To have an image appear in your post on the Forum you need to tell the Forum software where you want the image to display in your post and where to get the image the you want to display. Depending on where the image is located on the internet as to how easy this can be, however hopefully this article will give you the basics to understand what is going.
Where to Display your Image.
You can place images anywhere in your post, before the text, after the text or even in the middle of the text, you just have to place special code, BBCode(Bulletin Board Code), where you want the image to display so that the Forum software does extra processing to display your image and not think what you have written is just text to display in your post.
BBcodes can do a lot more than just insert your image, many of the editor buttons you might use when creating your Post are just inserting BBcode for you to highlight or change the way your text looks when it is displayed on the forum. BBcode is used in various forms across other Forums and similar services on the Internet and you can read more about them here with examples.
BBcodes are found in pairs of Square Brackets '[' & ']' and there will be a starting pair and an ending pair, however the ending pair will also have a slash '/'. Within the  you place the Type or Mark up you want to use and between the pairs you place what you want to process. The format is [type]what to process or mark up[/end this type].
Some common BBcode you will deal with in inserting images on the Forum are;
[IMG]location of your image[/IMG]
[URL]Link to your image[/URL]
How you would use it in your post is to have something like;
and when displayed on the forum you will see this;
Where your image is located
Every image the appears in your post must be stored somewhere on the Internet and for it to appear in your Forum post you have to place the location of your image in between the [IMG] & [/IMG] tags. The location or your image, the URL, normally takes the form of http://www.somesite.com/mypicture.jpg and depending on where the image is stored as to how you can get the URL.
Common ways are to right click on the image and select 'Copy Image location' or right Click the image and select View Image and then when the image is displayed copy the URL from the top of your Broswer address bar. Services like Photobucket might give you a button or selection to get the location of your image.
Once you have got the URL for your image you can Paste the link into your post and place the tags [IMG] & [/IMG] at each end of it or use the Image button in the Forum editor and paste the URL into the URL box that pops up as shown below.
Again depending on where your image is stored, URL when you Paste it may contain other stuff after the name of your image then you should remove this excess information to help display your image properly. Your image should finish with .jpg, .gif, .png or .jpeg so in most cases deleting anything after this should work for you.
Place the cursor where you want the Image to appear and then select the Insert button next t the URL you have just pasted.