If you make it yourself and you dont mind others using it, do GPL / GNU. If you borrowed free-artwork from someone else, you must release it under the liscense of the work you borrowed from.

For example: If you borrowed the KDE letters from another picture, you are legally obligated to find the original liscense and publish yours under the same or stricter liscense.

If you borrow a microsoft logo, you cannot publish it on this site (although people often do), since microsoft holds a strict liscense on all of their material.

If you make it yourself with photoshop or gimp, then you can release it under whatever you wish, and GPL / GNU is the best one for the free world!

Cheers.

-FBL