Absolutely under no circumstances can you upload a pattern that you do not own the rights to. It does not matter if it is a free pattern from another site. Do not upload it. If you want to tell the members of the forum where they can find the pattern that's fine. If you created the pattern make sure you are not violating any trademark or copyrights. For example, uploading a pattern of the Harley Davidson emblem is a no-no. Disney characters, school mascots etc... are all on the forbidden list.
This is a very serious matter for me. I am the one that has to answer the Cease and Desist letter from the lawyer. They go something like this.
Dear Mr Good, it has come to my clients attention that your web site is in violation of his copyrighted material. We demand that you discontinue any further use of this material and submit back to us the number of times it was viewed so we can calculate our financial damages. To save all parties the time and cost associated with legal action we expect an immediate reply.
I have seen these letters twice. Both times they were because of an unintentional mistake and fortunately the people on the other end were easy to work with and all went well. They could have worked out much worse.
I know none of you do these things intentionally but I have to let everyone know what the rules are.
Remember "The beatings will continue until morale improves."