That image is static, as I see. 23c0 in PPU - that's your place to look for the attribute table. Depending on the game (and actually quite often) that table is placed in the ROM in the exact way it is PPU. In some cases you will have to do some asm to apply changes - it could MTEd for example.
PS: Read all the answers in this thread once again - I think, you way out is already there.

UPDATE: Yeah, you were right - it's not a usual case - tried to look it through and it seems that the attributes really go through a long way...

LAST UPDATE: Ok, turned out quite simple after all. You've chosen a wrong place to seek from the start. I've done that debugging sequence and that's what I've found your table - non-compressed.
If you are not sure, that you've found the correct place for the table look for the IMAGE itself in PPU. Attribute table usually goes BELOW it. In your case, it was 27c0.
Set a write break point and voila - all that goes next is quite simple and plain:


Your table starts at b933 - play with that a bit to get the result you need.  A little hint - the attribute you will probably have to change is 44 (after a bunch of zeroes). For this I've used the US version of the rom, but I think the PAL one should be the same. 

Firstly, I would like to say sorry for taking so long to answer. University life without internet is driving me crazy.

Secondly, I would like to say thanks for both of you! ashsha's idea would be easily executed, as my friend who is working on this ROM made a wonderful job with tilemaps to be able to write "LOBISOMEM" where originally was "WEREWOLF". He knew the offset of this screen, and using this logic (that the attribute table comes after the image itself [and I knew that, but had forgot ...]) I could easily look for the 16th attribute byte and change it to what I wanted.

But tummai also noted a small detail that I overlooked and that made all the difference: the nametable where the image has been constructed. If I had opened the nametable viewer, I wouldn't had to re-open this topic.

Still, thanks again, guys! This topic means so much to me