Firstly the photo must be available online, so if it's only on yr computer load it up to a site like tinypic. There you select the layout jpg address. If an image is available online click on view image and select the address ending in .jpg .gif or .png. On steemit if you use the Editor -right hand corner when you start a new post- you select the image icon, which opens a popup where you can paste the image address. If you don't use the editor type something like this exclamation mark, square opening bracket, square closing bracket, normal opening bracket, address.jpg, closing normal bracket.
Hope this helps. Good luck.