Looks like you're almost there, just selecting the wrong photos.
It's slightly more difficult to post pictures when replying to an existing thread as opposed to when you're starting a new thread.
You'll first want to hit "Go Advanced" at the bottom right corner of the area where you're typing your message.
Next, scroll down underneath the smiley's to where it says "Manage Attachments" and hit that button.
When you hit "Choose File" it should open a new window prompting you to select the photos you want to post.
It's a little trickier to select the right pictures if you don't have preview images displayed or file names that you can recognize (so you know what pictures you're selecting).
I've always used Windows so I'm not sure if it's different on a Mac, but after you get to your PHOTOS folder try right clicking in the background (not on a photo itself) and expand "view" and change it to extra large icons, or something like that. That way you can see a preview of the picture to make sure you're picking the right one. Then you'll know if you're picking coin photos or cactus photos
Second step, if that doesn't work, navigate to your PHOTOS folder outside of treasurenet, find the picture you're trying to upload, and change the name to something you'll recognize.