Hit the post reply button and look down towards the middle left of the page there is a button that says manage attachments listed under additional options.
Miscellaneous Options
Show your signature
Automatically parse links in text
Disable smilies in text
Attach Files
Valid file extensions: bmp doc gif jpe jpeg jpg pdf png psd txt zip
MANAGE ATTACHMENTS
Thread SubscriptionNotification Type:
Hit that button and navigate to the folder on your hard drive where the image is stored. The forum software will try to compress the image if it is too large but if it is below 200000 mb it can be uploaded. Hit upload and submiit reply.
P.S.
To compress images to manageable sizes some people use
bluefive, a freeware tool that is clean (meaning no bundled spam with the download).
The one you want to download to your desktop is called picresizer 2.0.
Normally you wont need it but with camera shots they are initially very large before they are compressed.
A good max forum size is generally 600 x 600
larger than 800 wide will make the page scroll right and take up unecessary bandwidth.