I've recently migrated to Apple's new Photos app and uploaded all my pictures to iCloud. I thought I'd experiment and see if I could use that service to host photos for forums, like this one. The process:
You need to have iCloud Sharing turned on.You can do this in either the Photos app preferences or the System Preferences. Photos is easiest as the option is right there on the iCloud tab, at the bottom.
Create an album to share.Click on the SHARED tab in Photos and then the + sign at the top right. Be careful what you name it as you can't change this later! Don't worry about inviting people (that is only for sharing with others using the Photos app themselves), just click Create.
Add photos to this Shared Album.In the Photos app simply select the photos you want and click the Share button (a box with an arrow pointing up from it), at the top right of the app. Here you select iCloud Photo Sharing and then the album you created.
Make the Shared Album public.Go back into your Shared Album (under the main SHARED tab and click into your album) and now there is a new icon at the top right with a little circle containing a face. Click here and select Public Website. The web address for your new Shared Album now appears.
Select the image you want to post.Go to the web address you just created and click on whichever photo you want to post. Now right click on the image and select Copy Image Address.
Add to your post.You can now paste that image address into your forum posts. It needs to be between [*img] and [/img*] tags (without the *). You can pre-generate these tags by clicking the Picture Icon above your Post Reply dialogue.