Wrong Args.
Use
skin.miniminers.de/[type]/[size]/[name](/Armor)(.png)
[bla] = needed, (bla) = optimal
Type = FULL || HEAD
Size = 1-512
If you add /Armor at the end it will display armor too
If you need you can add .png at the end, it will only affect the url.
Examples:

http://skin.miniminers.de/head/64/Paulomart
http://skin.miniminers.de/full/32/Paulomart/Armor
http://skin.miniminers.de/head/128/Paulomart.png