Cannot upload specific .png file to thread

This has happened to me before, and I… I just don't know, man.
This newest time, I was trying to post todays art piece to my Sketch Thread, created the art piece in FireAlpaca, saved it as a transparent .png file, and tried to upload the file, so I could post it, and it just doesn't do it. It does the little loading thing, and then it stops, and nothing is uploaded.
See, here's the image saved as a 24-bit PNG instead (something without transparency)

And it uploads fine. I'll have to post it in this form so I can at least get it out, but if I could get some help avoiding this in the future, that would be great.