Well I just figured it out, and what a stupid flaw in Apple's system this is.
I googled around for a bit, and apparently iMessages won't send pics if you have wifi on; it will only send if you have cellular data enabled.
Since I am tethering from my iPhone right now, I figured it's the same issue. I turned off personal hotspot and tried again and it went through without any hiccups.
But seriously, major fail on Apple's part. Doesn't make sense.
Edit
I suppose I shouldn't be so quick to slander the Empire. . . I reset network settings and now everything works great.
😱