After a little more digging around, I came across this thread:

http://www.dotnetnuke.com/Community/ForumsDotNetNuke/tabid/795/forumid/46/threadid/89710/scope/posts/Default.aspx

that talks about problems with PDFs and compression. I added the mime types mentioned there to the compression config file and the media module now displays images correctly with compression enabled - but only with the whitespace filter off.

In short, I added the following to Compression.config:

<mimeType>image/png</mimeType>    
<mimeType>application/zip</mimeType>    
<mimeType>application/x-zip-compressed</mimeType>    
<mimeType>application/x-gzip-compressed</mimeType>    
<mimeType>application/x-compressed</mimeType>    
<mimeType>application/octet-stream</mimeType>    
<mimeType>application/pdf</mimeType>

Just wondering whether this problem has been resolved other than just turning off whitespace filtering?

I have a module that stores and retrieves images from the database rather than file system and the mime types don't seem to have an affect and still don't get excluded.

The page executes and retrieves as image but after filtering its no longer a valid image so won't display.

Any suggestions?

Thanks
Lucas