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Basically the texture coords are wrapping round; repeating instead of clamping.
You'll notice the undesired artifacts on the right and bottom sides are actually the same as their opposite sides.
Incidentally you should knows that you're probably better off using texture dimensions which are of a power of two (e.g. 16x32... 512x512 etc) as often a PC's graphics card will upscale them to the nearest power of two anyway which will cause nasty artifacts.
As for solving the wrapping problem i think you can change the texture's wrap mode from repeat to clamp somehow, not sure how tho!
From all i understood, changing a texture's wrap thanks to SMaterialLayer is possible only with an ISceneNode.. But i'm adding a texture from the gui environment so i can't do this stuff..
Anyway i found a 'solution', by setting to true setImageScale of my image. Not that i like scaling but sounds that no choice.