While in your profile and in the "Signature Details" section, click on the 'Insert Image' icon (at the far right of the icons).

Click on the 'Browse' button and then select the file or pic you like from your own directory.

Then click on the 'Add Image' button.  If the image you've selected is larger than 500 X 500, the system will automatically downsize it to this size.  If you'd like it smaller, then you would need to do this before downloading via this method.

Just exit your profile and go to any of your previous posts and the image "should" be there.  No guarantees on this.

Paul,  Does anything happen when you click on the 'add image' icon?  You should get a popup window that has among other things a 'browse' button and a 'submit' button.  I took the liberty of moving your picture to my own directory and it uploaded okay from my end back into your profile.