For me (Windows 7 and Firefox browser), when creating a new thread, toward the bottom in the section called Additional Options there is a button called Manage Attachments. A new window appears (you may have to set your browser to allow this). Then I click the top Browse button (in the section labeled Upload Files from your Computer) and find the picture that I want to upload, click on it and then the Open button. It will return to that new window. You should see the file name of the picture that you selected in the top section. Then click the Upload button to the right (and slightly below). Now the file name will appear in the center section (labeled Current Attachments). Hit the Close this window button or select another file to upload.
This should return you to your original browser window. You should see the file that you uploaded listed. Then finish your post and your picture should be there.
Hope this helps.