Rule one, it really needs to be a JPG file. An PNG file will never work, neither will the dozens of other formats.
If you have a Windows computer, there is a Pictures folder already. Make a folder in the pictures folder and call it something like Pelican. Put any photo you want to upload to Pelican in there.
Once you have a picture called whatever.jpg look at the bottom of the post box and you will see a large "Go Advanced" button.
Once you go into Go Advanced, there is a large blue bar that says "Click Here to Upload Photos" so click it.
It will look like this. Drag the photo to that window, or click once in the window, and it will bring up the file folder info and you can find the photo or photos you want to upload. Just select up to 10 images, and it will upload them. Then click preview post at the bottom of the window and you can see if it worked. Then post it.
Done!