Pretty nice i have a new background but i don't know if its allowed.
In general, try to keep more dubious content to the sub-forums that are not publicly visible (Creative::Artwork and Creative::Authors).
Then, if it's adult, host it on another site and just put the link in the post (don't upload it to PSF or embed the image), and clearly mark the link as such.
If it's //really// adult -- "offensive", if you will -- then yeah, consider avoiding posting it altogether.
That said, if it's only slightly questionable -- some moderately revealing clothing or something; something your kid sister could catch you looking at without either party getting concerned -- it should be okay to link to it from this sub-forum too.
If you're not sure where it fits on the spectrum, you can always PM Darrin or Smiles for an adjudication.