Try uploading your pictures first to the gallery here on site, and linking to that address. If that works, then you know your picture is acceptable in size, at least.

There was always confusion about uploading pictures from within the profile...which option to use when.

OR try this:

RIGHT CLICK on the picture you like
The drop down will have many choices such as :
Copy link location
Copy image
Copy image location

Select "Copy image location"
Then, in the post area, click on the yellow box with the moon and mountains in it on the task line/bar above the window.

Hit Ctl V to paste what you just copied (image location)
Hit Enter
That should put most pictures into a post.

If that doesn't work, you may need to copy image into your own computer's gallery first. Then link to that.