Utilize the "Upload image to post" link just below the text window when you type a post and then follow the instructions on the window that pops up (drag and drop or search for your photo on your computer or mobile device):
If you have an imgbb.com account, you will be able to decide which album the photo goes into. If you don't have an imgbb.com account, it should upload anonymously. Anonymous photos cannot be easily removed from the site, so use this option with care.
I'm working on a sticky to show people the finer details of using imgbb, but it is pretty self explanatory and quite easy to use!