If you are kind of new at this then...
A real easy way to do this is to move or copy the picture to your desktop for easy access. Sign up for a Photobucket account. Click on "Upload a Photo" then "Select Photo or video" this will bring up a browsing window into your computer. On the left select "Destktop" then just click on your picture that you want to upload. Once it is uploaded you will want to click on the "Save to album". Once this is done you will see the picture on the PB page. If you hover your mouse over the picture it will bring up a list of links. Click on the IMG Code link and it will be automatically copied. Then all you have to do is Right click and "Paste" (or ctrl + V) into the post you are making on the Forum. When you hit the "Post" button on the Forum your post will show the picture selected in the post you made.