From the advanced reply box in the forums, you'll select insert/edit images.
Click the file icon to bring up the Upload/Manager for Images and Files
Select Upload and from the drop down select Local Machine. Now select which file/image you would like to upload from your computer.
After it's uploaded to your file manager, select and insert the image.
Note: To edit your image select Edit from the Manage drop down menu. You can alter the image size and apply filters from this menu.
Your image is all set to insert, click OK.