Hey Lio.
I don't know how it works on a mac, but I can tell you how it works on a Microsoft PC.
Finding Your Screenshots
Search "%appdata%" (without the quotes) This will bring you to a folder page named "Roaming."
It should have a folder titled ".minecraft" Open that folder.
In that folder, there should be a folder titled "screenshots" Open that folder.
Inside that folder is all your screenshots. Keep that window open.
Posting Your Screenshots
Click "Attach File" at the bottom-right corner of the text box on the forums. This will open the standard "upload" window.
Take the other window from earlier with the screenshots. Drag one to the "upload" folder.
That screenshot should appear in the "upload" folder. Select it.
Click "Open". It should start uploading. This process may take several seconds.
I'm sorry that I don't know how it works on a mac, but you could try this.