My questions are:
The word use in relation to images implies distribution IMHO. For example if I 'use' Tux in a pamplet, i am most certainly distributing him; however, his stament makes it quite clear that that is allowed.* does this suffice as a DFSG-free license? it seems that there's no explicit permission to distribute (even if I suppose Larry meant to give such a permission...)
* is this a GPL-compatible license? there's a little restriction
("acknowledge the GIMP, if someone asks") that I'm not sure can be
construed as present in the GPL
No, but make the statement part of a copyright notice, and it cannot be removed according to the GPL.
ie:Tux Copyright ???? [i don't know the year off-hand] Larry Ewing <lewing@isc.tamu.edu>, created by the gimp.
There it is acknowleged regardless of whether anybody asks.Also remember that we currently distribute Tux with the linux kernel (GPL, of course). (Booting in frambuffer mode should show one tux per installed processor. [on a few archs iirc Tux is not shown, but is replaced with a different image :(]).