While I'm sure easily uploading photos is in the development pipeline.
Some ways to do it:
If the picture is already somewhere online, lets use facebook as an example...right click the photo and select 'open image in a new tab.' Click into the 'new tab' if it doesn't already switch for you and copy the URL at the top of your screen. Paste this into your steemit post and check if it worked in the preview below. [Note: This method doesn't always work. Sometimes the image are prevented from being 'hotlinked' like this.']
[My Personal Method] Goto http://www.imgur.com and click the green new post button at the top. A window will pop up that will let you either upload a photo or provide a URL (just like you got in option 1.) After you've uploaded the photo you'll see you're newly uploaded image in imgur. To be sure wait about 5 seconds to let processing complete, then right click this new photo and again select 'open image in a new tab.' Like before in this new tab copy the URL and paste it into your steemit post.
If you have any problems or other questions feel free to join the Steemprentice Mentor Group. We have some good starter resources and a bunch of people only there to help and give feedback.
Keep Smilin and Steem On :D