This is the COMPUTER (PC) method. If you are using your phone you will have to wait for someone else. This is not a "one and done" process that some of the social media site have.
First the picture needs to be stored somewhere (and you need to know where that "somewhere" is) on the computer.
Note that there is a picture file maximum size and natively out of the camera/phone that picture size can often be too large. What I do is open the picture, then use the Snipping Tool to recapture the image and it will create a new picture file that is smaller in data size that you save. I recommend you use some indicator ( I use LR) in the file name to indicate it is a "lower resolution" image.
On the page where you create the post scroll down to "Attach." Select "Choose File." This will open your folder files. Navigate to the location (drive, folder, image) where the re-sized pictures are located. Click on the picture and the picture file name should fill the dialog box near the bottom of the open window. Click on "Open". That should complete the process.
In summary: Transfer pictures to the computer, resize picture to an accepted size, select "Choose File" and navigate to the location the picture was stored, click on the picture to highlight it (and fill the dialog box) and select "Open."