The best way I found is this, go to www.photobucket.com and open a free account. Photobucket is a picture hosting website.

After you open the account, upload your photos via their process, it's very easy. You click browse, select your photo from it's location and upload it.

Once uploaded you can copy the URL, the one you want to select is the Direct Link, second one down in the list of 4. Click it and it autocopies or right click it and choose copy.

Come back to your post, I usually open two windows, one for HNI and one for Photobucket so you can go back and forth.

Type your message and then with your cursor in place, click the add image icon seen here in your post tool bar.

Delete the http:// that is there and paste your URL that you copied from Photobucket.

Click okay and bam, your photo appears.

Recommendations:

You can edit your photo to the proper size for a forum or an avatar.