Your pictures are 2880 x 2160 pixels and very few people, if any, have their monitor resolution set to anything near that width and height.
Using the paperclip on the top row next to the smiley face to attach the pictures to your posts solves the problem. Only thumbnail size pictures are seen attached to the post and when they are clicked on by the reader, they open to the max resolution of the monitor they are being viewed on. If a more detailed view is desired by the reader, left clicking in the image will display it full sized and the image can be scrolled to bring points of interest onscreen.
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Looks like several people beat me to posting pictures 101.
