Ok, here is my idea: a option when you post a thread comment that when you attach a file you can select an amount of time the file exists for. I know this may not be possible but I do not know the possibilities of development of the forums. I think this is a good idea as I run a skin company (sneaky bit of advertisement) which has no other way of sending players skins than attaching them to the thread, then later deleting them. This is risky as other players could download them which would be unfair for the one who paid for the design. If you had a window of time it existed for before being removed that would allow for a safer transaction (unfortunately with not being able to PM images it will never be fully safe) . When selecting a time you could type into a bar the amount of time in hours, or just leave it blank if you don't want to use the feature. I would like some input on this idea, but please refrain from aggravation.
We can't do stuff like this. You should upload it to imgur.com and then PM the url to it privately in PM.
Plus, there's nothing stopping people "stealing" skins directly from other players via Novaskin or similar stuff.