First off, there's two diffrent ways to post pictures, there's posting pictures that you found off the internet (things like google searches) and pictures that you have saved (ones on your hard drive).
Posting a picture from the internet
First you need to find your picture(I just got a quick one from google) and right click the picture, then go down to the properties, it should be at the bottom of the little menu. Click it.

After you clicked the properties button this window should pop up, go down to the adress and copy it (a quick way to select it is to double click the text and then copy it).

After you have that copied go into the comment box to the thread you're replying to and paste the link. In front of the link you will need an [ img ] and at the end a [ /img ] (no spaces). After you have put that in click the preview button next to the post button. If all went well the picture should appear above like it did in mine.
Posting a saved picture
Go to an image hosting website (I used imageshack for this) and click on the browse, a window should pop up that shows you the pictures you have saved; you may need to do some navigating to find where you saved the picture, once you find it double click the picture or simply hit the open button. After you've done that click the 'upload now' and wait for it to load to the next page.

When the page finishes loading you should see your picture and these text boxes below, go to the one that says 'forum code' and copy it, once again right clicking the text will select it all and you can copy it.

Now copy the picture you wanted in the comment box. The first line is the picture itself, the second line underlined in red is the part where it says 'uploaded with Imageshack.us' and you can delete it all together. The upside to posting a saved picture is not having to enter in the [img] yourself but can take more time due to browsing for the desired image.
Hope it helped.