I'm not sure I understand the question. DominoWiki doesn't do anything to pictures uploaded to the site.
It's just standard file upload/ attachment stuff, no different from any other Domino application. If you're
getting artifacts in the files you attach, I think you need to look elsewhere in your infrastructure. It sounds
like the file upload process isn't completing properly.
When you attach files in a Notes database, they are compressed as standard. From Notes 6.x, the only
user control around this is whether to use the Lempel-Zev algorithm or not (see the "Advanced" tab in
database properties for this tick-box). You can read more about LZ here:
http://www-1.ibm.com/support/docview.wss?uid=swg21090049