Well, you hit the "post reply" button at the bottom of the messege thread; when you're at the post reply screen, scroll down the bottom and you'll see a box that says " ATTACH FILE", a text box and a button that says "browse..."
You can enter a path that you know already ( i.e. you know the image is in C:\My Documents\ pictures\ or whatever as an example ) , or you can hit the "browse..." button and locate it that way.
Once the correct path is there, and you've added any messege you'd like to in the " Your reply" box, just hit " submit reply" and your lovely pictures shall grace this fine thread!
It sounds a lot more complicated that it is ( the way I'm explaining it anyway), so give it a try and you'll see it's actually really easy. Good luck with it!
...forgot to add, that the image has to be under 100k in size or it won't post.
Any questions just PM me ( click the little "PM" at the bottom of my posting, the PM screen pops up and you can write me a messege ) and I'll be more than happy to assist you in any way I can!