well, idk how i'd be able to help you, because i used the tiles on Tiled and everything worked fine, maybe you're doing something wrong
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I'm scaling them by 400% using resized.app. Maybe during that rescaling processing there is some pixel bleed, or something weird happening to the pixels. I checked your source in GIMP and it looks fine there. My apologies. I'll keep it at the original source and scale it using the SFML function. Hopefully that will work better.