It is certainly not appropriate to put any holy images on shoes or on carpets or the floor to be step on. While on the Buddhas' side, they certainly do not mind and Buddhas' are known to manifest as anything including a chair to be sat on if it benefit beings; it is from the side of beings that we need to consider the effects such act will bring.
For buddhist, such holy images printed on shoes does bring to mind that the Buddhas are being treated as lowly, and as mere decorations or exotic designs. For argument sake, any images of the Buddhas will created an imprint on the mind of sentient beings. However, couple with the effect of these images on shoes brings to mind to the uninitiated, that Buddhas are just motif or designs on shoes. Any object that that buddhist take refuge in should be highly regarded but to non buddhist, while just seeing the image of the Buddhas create an imprint on their mind; the overall effect is that the Buddhas are lowly and this cause more ignorance to already ignorant minds. So my suggestion is that, we educate the manufacturer and the customer that the Buddhas should be highly regarded. We should NOT go out and violently protest in a non-buddhist way. Remember, we are buddhist. Let show some class.
The antidote to this would be to create some hats with Buddhas images??