You will need to use a 3rd party picture hosting service (some have mobile apps), imgur.com is popular on this site, picpaste.com, gyazo.com, google photos, many others out there. Personally I pay for my own web hosting and post my photos from there, really doesn't matter in the grand scheme of things where you share them from. For this forum, it would probably be ideal to resize most of your photos down to a lower resolution such as 800x600, or 640x480 even, many users here are on metered or throttled mobile connections or older devices and may have trouble with large images.
Once you have the image shared on a photo sharing service, you can just use the "Advanced Post Form" here and use the insert picture button and paste in the photo URL from your service.