I know you can add wallpapers, but can I also modify the icons and the alert boxes? I love the TViX but the User Interface is so ugly I wanna add my own icons and stuff...

To an extent, yes.

You are supposed to have 3 main categories of media as assumed by Tvix: video, audio and photos. Each category is a different folder with subfolders. I have yet to change the icons for these 3 main category folders but all subsequent subfolders are pretty much replaceable.

For example, in my video folder I have 3 further subfolders: movies, tv series and music videos. Inside each folder I put the icon.tvix file and any icon that I wish to replace the mundane Tvix icon, I rename that to folder.png (make sure it is exactly 100 x 136 pixels in dimension, or else Tvix will ignore it). Similarly, put any icon you wish to replace the upfolder icon and rename it upfolder.png (or jpeg, as your case may be).

So to summarise, each modified folder would contain the following 3 files:
icon.tvix
folder.png OR folder.jpeg
upfolder.png OR upfolder.jpeg

I suggest deviantart for an enormous library of free-to-use icons. There are some incredibly talented people there and I love what I found.

thanks for the tips!
what kind of file is the icon.tvix file?
i get that the folder.png and the upfolder.png is for the folders in which the file is placed in.
but what for is the icon.tvix file?
Also it says in the manual when you have a png file thats named exactly like the media, does this make an icon for the movie?
And what is the deal with the wallpapers?
Sorry about all the asking, but the english in the manual is so bad i quite dont get it... ;)

Also I would love the find the image files from the firmware and chenge them. i.E make new alertwindows and icons for the top menubar (the harddisk, network and usb images)

nevermind, I found the answers in another thread