1) Create a folder in your library; open the folder. From that folder, upload the image to your library using the 'Upload a file:'.
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2) From the post (message) frame, select the library folder icon below the message box.
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This will open a tab to your library. Select the folder of interest.
3) Select the image (place the cursor over the radio button and click). This will place a link to the image in the message box.
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This should work.
Should you want to post an image from, or link to, an Internet source, use the 'Link' box at the bottom (for multiple links, see the FAQ).