For some reason, I post a image from here onto my own forum(with the "[img]" BBcode), the images shows up for a couple of hours or so and then apparently "expires" the next day.

Is Gelbooru's image database always being refreshed constantly or something?

Because I never had this problem a few months ago and I doubt it's my forum's fault.

Hotlinking has been recently disabled. Please do not use direct links to Gelbooru images, as they cause severe stress on our servers. Our servers were recently completely disabled just because people threw up some direct links to Gelbooru images on a website in China, so this is a serious problem.

However, feel free to post links to post pages (i.e., http://gelbooru.com/index.php?page=post&s=view&id=1 ) instead of direct links. Since users must intentionally click the link to load the image, it causes less stress on the servers; and if they click the link to view a proper post page, it can help generate ad revenue for the site.

Well, I can understand that it puts stress on the servers..

Unfortunately, most of the images don't have alternate links(pixiv,etc.)

But, thanks for notifying me, though.

You could also save the file and upload it to an image hoster that will allow it depending on the image. If you want people to see thumbnails instead of text links.

Of course, forums across the internet tend to be flooded with "no hotlinking" errors on images from those as well. It is an inevitable problem.

Most of the source sites probably have anti-hotlinking as well.