First, the picture must be in the Photos app. If you took it using the iPad, then it's is already there, in the camera roll.
If you are accessing the forum or other service using an app, the apps should have a way of uploading a pictures.
For instance, if you you are using this forum's app you can go to the Profile tab and tap on the avatar picture. You'll get the options of either using a picture from your library (the Photos app), or take a new one using the camera. To add a photo to a post use the Advanced button at the top right while creating/editing your post.
For other apps check with the developer's support site (found on the app's description page). Or, if it's a common app you can try asking here. Sometimes the answer is simply, no, you can't do it.
If you are accessing the forum or site using Safari, it should be possible to upload pictures using that site's normal methods. Pictures from the Photos app are the only files you can upload using Safari. Given the many different methods use to upload files on the web, some sites wont work. If the site offers different file upload options (this one does), choose the most basic.
There are other solutions (hit or miss) that can be used for other file types and problem websites. They need to be handled individually; though iCab Mobile, a third party web browser, is often the main ingredient.