You need to host the picture somewhere else, then link to it. I use Flickr, but there are others of course.
I think you can also upload them to this forum, but the size limits made it not hugely useful when I tried it a while back.
On my phone /tablet I use a "Photo resizer" app to make the photo small enough to attach straight to my post on here.
On the PC/laptop I used to use a photo hosting site like Photo bucket and copy the link.