This is Photobucket help page on uploading
http://support.photobucket.com/entries/21073236-Uploading-With-PhotobucketEven better, this is a video on how to upload on Photobucket and then post in a forum.
The person in the video has the picture you want to upload copied on the computer desktop.
Once you have it there, you just drag it.
If you prefer to upload navigating your folders just like you did for the attachments, instead of dragging the photo you click on the blue button saying "Choose photos and videos".
It will open a window where you will navigate to where your photo is. You can choose multiple photos keeping the Ctrl key pressed, but first try with just one.