[Mooff]

I see.

The addon does know you have GD installed and it is working (partly) as shown here:

Starting with images/sprites/sprite-49-main_rtl.png.
18 icons added.
Creating sprite (width:313px height:18px).
Sprite saved, updating css-rules.
CSS template successfully updated.

So it does sprite something.

As for the errors. That particular error does hit here:

Originally Posted by sprite.php_pseudo_code

else if(~imagetypes() & $imageinfo[2])
-> throw error, skip icon

If your gd version does not support the imagetype that image has, the addon cannot add that icon and therefore doesn't.
In your case i assume (unfortunately the error message doesn't show us which image type due to an oversight) that your gd version doesn't support *.gif files. So the fix would be to update your GD version.

Regards Mooff