Federelli, I think there is a relation between your problem and something strange I found
using extremely wide textures in SSB (a good example is the DATA Tab in the Data Section of the Main Menu): Either they are ignored, or they get corrupted when displayed. If that's the case, this is an old bug (which I forgot to mention), not new in beta10.
Some comments about this:
1) The texture I want to replace with is a 4X.
2) I tried on beta10 today to confirm if the bug still existed, and found that apart from not being displayed, the bug affects other textures as well (2nd screenshot).
3) In the earlier betas (and 6.1.0), that seemed to happen only with DirectX, OpenGL didn't have any bugs
EDIT: I added two more screenshots to confirm it (#4 for DirectX, #5 for OpenGL, sorry for the amount of images). Apparently, the same happens on both renderers now. Sometimes, the rightmost portion of the tab appears at the left, with wrong size.
4) My textures are RGBA, not CI.
5) The RAR includes the hires texture, in 4X and 2X versions.
Federelli, as a workaround,
try reducing the image to 2X, it worked for me (third screenshot). Compared with 4X, the sharpness loss is really noticeable at 1024x768 and up, but your texture might work fine this way, and it's still better than 1X.

I've never used 3X, anyways, I suggest you try with a 2X texture first. (Btw, looking great!)
Rice, sorry about not reporting this before. I forgot due to the workaround.

lain: