As with anything in life, there's lots of ways to do it! But here's what I do...
1. Upload you picture to a web accessable spot - either your gallery here on aquariumadvice.com, or photobucket, or wherever...
2. Navigate your browser to the picture you want to post, and copy the picture location to your clipboard (CTRL+C).
3. Go to where you make a post on the forum and where you want to add the picture, click on the "Img" button towards the top of the reply window. (It's right under the "subject" line.)
4. After you click on the "img" button, paste (CTRL+V) the picture's address, then click on the "img" button again.
You should now see something like this...
That should do it.
What kind of error are you getting? Depending on where you're "parking" your photo, some servers don't allow "hotlinks"... which means another website can't access the picture. It's done to save the host's resources and keeps people from highjacking/stealing photos.