SOLVED
I had big problems with this plug-in before getting it to work.
So hopefully I can help others by telling what I did:
1. Problem was that the image and text didn't show up at the registration page at all - solved by this post
http://forum.coppermine-gallery.net/index.php/topic,36319.msg225008.html#msg2250082. Problem was that image didn't show. Only a red cross. In the time I was searching for a solution my login timed out. So now I couldn't even log in again, because I couldn't see image. In the future maybe login confirmation should be disabled by default when installing.
3. Tried to delete the SQL table as mentioned in this post
http://forum.coppermine-gallery.net/index.php/topic,36319.msg170592.html#msg170592 - now I could log in again.
4. Changed the settings in file codebase.php so that this problem wouldn't occur again.
$CAPTCHA_DISABLE = array('login' => 'Administrators,Registered,Anonymous',´
'register' => 'Administrators,Registered,Anonymous',
'comment' => 'Administrators,Registered,Anonymous',
'report' => 'Administrators',
'ecard' => 'Administrators',
);
5. I tried Installing and Uninstalling the plug-in several times - no luck.
6. Found that you can put the code to a file instead of a picture.
Find this in captcha.class.php
function Create($sFilename = '') {
Change to
function Create($sFilename = 'test.jpg') {
This way i could check that the everything worked, because the code was actually made in the jpg file. Hmmmm OK thats one step further.
NOTE: File can be found in the root folder and the picture will NOT show until you remove this 'test.jpg' again. Only one is allowed at the same time - file OR picture.
I have no idea why this was working with the file, but not with the picture on the page itself. Maybe someone can answer this?
7. Looked through this huge thread trying to find a solution - at the moment it's 40 pages big. Found this post from Nibbler
2) Clean the output buffer just before the image is displayed.
captcha.php
Find:
$code = $oPhpCaptcha->Create();
Change to
ob_end_clean();
$code = $oPhpCaptcha->Create();
8. SUCCESS - now everything worked and now I just changed the configuration in codebase.php to the settings I would like to have on my page.
9. Attached is also the Danish translation of the confirmation messages - Huge job to translate

Hope that this is helpful to someone. I found this solution by searching the board!
Not saying it's the fastest solution, but remember that the members here is helping because they would like to help - NOT because they have to!!!