Version 3.1 of Sokoban supports a new skin format. Png, gif and jpg images as well as bmp can be used in a skin. Png makes the total skin size much smaller than the ones with bmp. This allows for larger bitmaps and nicer looking skins.
The new skin format also can have images for showing possible moves.
That's a nice improvement!
I hope that some creative skin designers can utilize the new skin format and make some nice skins.
Congratulations with the new PNG image option for the skins!
Unfortunately, in the process you have made a couple of serious graphical errors in the two best skins you have in the program. It affects your two own artistic high-quality skins, which now come as PNG versions with 100 pixels height:
"GreenMetallic" has the most serious error. The floor-color in the wall tiles doesn't match the floor color in the floor tiles, with the effect that there are ugly white stripes around the interior walls in a level.
"Classic" is fine, except for its "pack marker" (the "highlight-reachable-squares-image"). It looks like crap and certainly doesn't live up to the otherwise high-quality images in the skin.
The problem with the pack marker is that it has an outline which graphically looks like an antialiasing problem - not like a rendering of the physical "box" object. Instead, it looks as if there is an outline around an empty space, and inside that empty space there is a blue marble.
This can surely be made better. For comparison, you can try to download a new version of my "Sokoban YASC" program, where version 1.535 can import your new PNG-image skins.
In YASC, you import your new version of "Classic", and then click a box in the loaded level. Then you'll notice that the program highlights the reachable squares with a beautiful semi-transparent version of the box, which really looks like the box. It has no resemblance to the ugly "pack marker" image in the Sokoban 3.1.2 version program.
Another small issue, which isn't related to skins is the term "imbed" which you use consistently in the documentation. Strictly speaking it's a correct term, but it's problematic to use it because it's unusual, and many people (like I) probably wonder if it's a spelling error. It's better to change it to "embed", which is the the normally used form of the word.
There is also a bug in the renderer which is visible when the "Classic" skin is loaded. Try to perform some moves including some where the pusher walks over a goal square and away again. Then undo all moves and use the "Play forward" button (">") at the bottom of the screen to replay the moves. When the pusher now enters and exits a goal square, the red goal square background "bleeds" into the neighboring non-goal square.