Hi! You can add images to a post like this:
Go to wherever you uploaded your image, then right click the picture. There should be an option that says something like "Show image", click that (works in Mozilla, not sure about other browsers). This should open your image by itself with a web address that ends with .jpg. Copy that from the address bar.
Now you can edit your post and choose the little icon for adding a picture, clicking that opens a bar for writing the url address of the image. Paste your image address there and it should show up on your post.