Preface: I've looked at some of the other posts I thought to be similar, and didn't find a solution.
I did all of this in Paint.net, edited the original file for left sword, and made sure it was in PNG format. I also, upon suggestion by a similar post, hedged my bet and took the new design and copy-pasted it over the original file for left sword and re-saved as PNG again. It isn't, to my knowledge, an upload error as it shows up just fine on Imgur and all other edits I've done thus far and uploaded to Imgur have worked fine.
As you can see, somehow this made the left blade invisible, and also turned parts of my gas tank bright pink for some reason -_-
I've love some help in this so I can finish up my costume D: thanks!
PICTURES OF BUG:
PICTURE OF EDITED LEFTSWORD FILE: