Hi Kenny1001,

I love this theme, and it's made me delve more into my N900 then I ever have before - A very nice side effect. I've changed the lockslider image several times, and I use the following process;

1. I use OpenSSH on the N900 and WINSCP on my home computer so I can "log" into the N900 easily (pretty sure you have to have root access on the N900, but can't quite remember).
2. Create your image but it does need to be 148x94 (148 wide, 94 high)
3. Get into the following folder - opt/usr/share/themes/M4riNa-D4rK/images/. This is where the "icon" or graphic is located and it's specifically called LockSliderThumb.png. It's not in any of the icon folders, so this is why I call it a graphic rather then an icon.
4. Change the original graphic to LockSliderThumb_Original.png. This way you always have the original that came with the theme.
5. Copy your new graphic into the folder and make sure you call it LockSliderThumb.png. Capitals and everything.

5(a). Make sure the permissions are set correct (CHMOD). I original had it set as R & W for Owner and just R for both Group and Other, but I have now changed it to R,W & X for all three (0777). Don't know if it matters, because when I first copied it across with the original setting it worked fine, so skip this step if it all works fine (this is a good reason to use OpenSSH and WINSCP as it makes it so easy).

6. Reboot the phone and you should be right to go.

I realise that this may seem to be a very noob set of instructions, but this is essentially what I am (a noob). Hope I helped.

Chris
(A noob that just happens to know a couple of things)