Another user was creating flag-avatars for the forums at request, so I requested an English flag (without the word England, which he had been doing with all others).
Then I went a-googling and found a website dedicated to Metal Slug Sprites, which had LOTS of animated sprites (the website is now gone and most similar websites now just have stills or spritesheets). I downloaded a martian ufo.
I then went a-googling for an online .gif maker, found one, uploaded both images, stuck the ufo on the top of the flag, and done.
So, I created none of the content, but I requested/searched for them and stuck them together using an online tool.