Well, that's purely network setup (nothing specific to Ubooquity), so it really depends on your OS and network hardware.
Try taking a look at the following threads, they might give you ideas as to what settings need tweaking.

http://ubooquity.userecho.com/topic/626428-ubooquity-outside-locak-network-setup/
http://ubooquity.userecho.com/topic/701174-for-some-who-might-be-having-problems-serving-over-the-internet/
http://ubooquity.userecho.com/topic/526455-access-ubookquity-through-internet/

I'll need a bit more information to guess what's happening.
By "incorrect", you mean the cover of another book/comic ? Or a page of the right book but not the cover ? Or something else ?
Also, on what are you running Ubooquity ? A desktop (Windows, Linux, Mac OS ?), a NAS ? Are your file located on the same device or do you access them from a network drive ?

If you can, copy and paste here the your system infos (there is a link at the top of your admin page (if you use the web admin), and a button on the first screen of the GUI ("General"->"System info")).

That's the problem with big "Download" buttons. People tend to ignore the small lines above it. ;)
Glad it's working now.

Thanks for your feedback !

That's an intentional limitation of the free version of the JPedal library Ubooquity uses to render PDF books (comics are rendered by a different one which just extract images without rendering anything else). Despite a very long search, I was not able to find a free Java library that was capable of correctly rendering all PDF files, so I settled for this one.

I have plans to try to use Mozilla's PDF.js library to do PDF books rendering. It's quite good and would greatly improve quality. But I don't know yet when I'll integrate it (and if it will work).

Yes, you can fix that by editing the theme you use (or create a new one if you use the default theme).
Add the following lines in the comics.css file:

.thumb img
{
    max-width: [[COMIC_W]]px;
    max-height: [[COMIC_H]]px;
}

I'll add it by default in the next release.

I have been able to reproduce the problem with the image you provided, I'll investigate.
It does not seem new though, as I had the same result with version 1.7.6.

Your first error is a different one, I'll need the image file to investigate this one too.
If you want you can send it to me using the "private message" option (just under the search field).