@CharlyDasher  
I don’t think it’s the same file. Besides the problem with the transparency, this version is 1920x1080, and the DeviantArt version is 3840x2160. I’ve never used SFM, but maybe you uploaded by mistake a “temporary” file here instead of the final render or something? I don’t know. Anyway, for what it’s worth, the only affected pics are this one, >>1464123 (merged), and >>1463997 (merged). Everything else looks OK, so it’s not a problem with the PNG format, it’s something specific with just a few pics, so try to figure out what was different with these pics.
 
And if for any reason the pics keep coming out broken, there’s a quick fix if you don’t want to use Photoshop or Gimp or whatever: just open the pic on Paint, and save it (not “save as”, just “save”). Just like that, it will remove the alpha channel, and the resulting image will have the problem solved.

@Background Pony #FF5C  
No, it wasn’t. I save most of my pictures in PNG when exporting it from SFM then I upload it to both DA and Derpibooru, but it’s the same file! Maybe I shouldn’t save it in PNG but rather JPG?

I think you posted the wrong pic, this version is half transparent and it looks super creepy when the page has a dark background. The version in DeviantArt if fully opaque and it doesn’t look at all like this. Was it intentional?