When writing a new post or topic, click on the Upload attachment tab in the lower left, there you can browse your local storage and choose what to upload.
Attachment TabThe maximum size per attachment is 128 MB and there is a limit of 10 attachments per post. Alternatively you can use a third party file hoster like:
google-drive, Dropbox, WeTransfer
How to report inappropriate topics/posts?
If you find a topic/post that you think is inappropriate, in the wrong forum or simply spam, you can report it so a moderator can take action by clicking on the exclamation mark on top of the post:
I can not find my post/topic/thread!?
When you are logged in you can find all topics you contributed to by clicking on View your posts in the navigation bar on the top of the page:
Find your posts
What is the meaning of the different icons in the forum?
Why can I not log in to the forums using my http://www.factorio.com credentials?
Your http://forums.factorio.com account is completely independent from your http://www.factorio.com account.
Why can I not log in to the mod portal at http://mods.factorio.com using my forum credentials?
The mod portal uses your http://www.factorio.com account.
If you think there are often asked questions missing here, then contact a moderator and send him Q and A.