Lots of manga in these site have covers that don't look like the main manga content
like
http://www.mangaupdates.com/series.html?id=35

i think you just take the last cover out there, i think a cover with the main character o something like that will be better

Well all covers are added my members (Technically someone with an updater or higher rank account).
If you feel it's not the best cover to convey the series you can change it .... I am unsure if it has to be cover for the last volume only.
I think as long as the cover follows these rules it should be fine.

Again I am unsure about the cover needing to be the last cover. (I'm sure a mod will clear this part up sooner or later)

-Sidenote: If you're wondering how to be an updater look at FAQ 😛 and while you're add it read all of the FAQ page.

Although I think a series like bleach would have the cover image locked ...

manga name

Yes yes, I know I make longwinded comments, but that's just me >.<
I should proof read my comments more, but I won't...
So keep in mind I'm filled with typos

Check out FAQ and Forum Rules if you haven't yet.
For errors linking in threads

There currently are no other rules for the series images. However, they tend to either be the cover of the last or the first published volume.

A just ruler amongst tyrants

Like lamb said, there is no concrete rule, but I prefer the cover of the first volume. Most of the time the cover does not contain spoilers and the cover usually shows what the comic is about (contains the main character(s)/theme/items/etc.).

Questions? Read FAQ before sending a pm to an administrator