To post an image to the forum, the image will need to be hosted somewhere first. One option is to use a hosting service like imgur.com, or you may host the image directly on this site.
The best method is to use MopedArmy.com directly to host your images. Images hosted elsewhere may disappear as fly-by-night hosting companies come and go, so will your images. MopedArmy has always and will always exist, so when you upload the image here, the pictures of your seized up rings will be here forever for future generations to see. It's no fun to find an old thread that sounds like it may contain the info you are looking for only to discover all the images have disappeared.
When using imgur, once the image is upload you'll want to copy the BBCode code for that image which will look something like [IMG]http://i.imgur.com/zzzzzzzz.jpg[/IMG]. You may now use this code to embed your image in a post.
Using MopedArmy.com Directly
If you wish to use MopedArmy.com directly, click the Attach a file link to the left of the main composition area. Click 'Choose File', select the file on your computer. Now that you have selected the file you need to click the Attach button. The image is now uploaded although it will only be displayed as an attachment to your post.
Next, type some stuff in the main text area. Even if you wish to just post an image, you'll first have to type something in there like, "blah". Click the Post Message button to submit everything.
Now, go back to the post you just made and right-click the link to your attachment and select Copy Link URL (or something like that depending on your browser). Now click the Edit link below your post.
Once back on the edit screen, click the link above the main text area and paste (ctrl-v or cmd-v depending on your operating system) to insert the URL you just copied in to the alert box that appears.
Save the post and you should now see your image embedded.
A video depicting the entire process may be seen here:
How to upload and embed an image on MopedArmy.com
MopedArmy's forum software will accept video links from several sources including YouTube and Vimeo.
To embed your video, go to the page on which your video is hosted and copy the URL in your browsers address bar. Click the link in the toolbar above the main text area. In the box that pops up, paste in the address that you previously copied, but make sure to only use http and not https. If the address contains https, simply delete the 'S'. That should be it!