You need to know the location of the jpeg on your hard drive.
When you click on the "Reply" button at the bottom of a post, a new window opens, labeled "Post Reply," where you will find a large text box available for writing your reply. Below that text box, under "Additional Options..." you will see a grayed-out box labeled "Attach:" with a "Browse" button at the right.
Click on the "Browse" button, and a file browser of your local computer will appear. You need to navigate through that browser to find the jpeg file you wish to attach. Once you click on the file in the browser and click the "Open" button, the path to that file will appear in the previously grayed-out box. Then, when you click on the "Post" button, your file will be uploaded to the LuLa server along with whatever you wrote in the text box.
If you want to attach additional photos (up to a maximum of four in any one post), click on "(more attachments)."
This is exactly what I did on my PC to attach this photo of flowers in my yard. I hope this helps.