1) Host your images remotely on a service such as Photobucket.com. Those sites provide the forum code that lets you embed an image within thetags.Code:[img]...[/img]
The advantage to using this method is you aren't limited to attaching only small files.
2) Another way to post an image from another site is to copy the image URL (right-click, then "copy image location" or equivalent), then use the post editor's "Insert Image" button...
A dialog box will open...
2) Upload your image using the "Manage Attachments" feature...
This also allows you to upload multiple images. (You can upload single images by using the "From Computer" tab shown in the step above.)
a) The "Manage Attachments" button appears beneath the post editor only when you start a new thread, or when you "go advanced" then "preview" a reply. It does not appear with the "quick reply" editor.
b) Browse for the image(s) on your computer, click "upload". Successful uploads will appear at the bottom of the dialog box:
The uploads (clickable thumbnails) will appear at the end of your post after you submit it.
c) Close the dialog when finished.