Machs mal so:
C
if(newkeys & KEY_JUMP)
{
static const skins[] = {1, 2, 3, 4, 5, 6, 7, 8, 18, 19}; //Hier kannst du alle Skinids reinschreiben, die zu wählen sind.
static idx[MAX_PLAYERS char];
SetPlayerSkin(playerid, skins[idx{playerid}]);
idx{playerid} = (idx{playerid} == sizeof(skins)-1)?0:idx{playerid}+1;
return 1;
}