One day you’re going to sneak a rootkit into these Flash animations and nobody will be none the wiser.

Impossible, for Flash is the most secure platform that has ever existed in the history of computing.

MS Paint's as reliable and serviceable as most of the many paint programs designed in the enduring mold of PCPaint and PC Paintbrush. I've occasionally used it in a professional capacity to tidy errant pixels in large images of photos or pages, and never suffered a glitch. Its limited functions are as flexibly and intuitively implemented as one could expect.

When I edit graphics and don't need or care to launch GIMP, I use XPaint or mtPaint, or their respectively forked variants, GPaint and rgbPaint. http://sf-xpaint.sourceforge.net/ https://www.gnu.org/software/gpaint/ http://mtpaint.sourceforge.net/ http://mtpaint.sourceforge.net/rgbpaint.html

I have to say, those are some genuinely well-designed Web sites you linked to. Unfortunately for me, it seems that XPaint and Gpaint don't work on Windows. (XPaint is even listed as compatible with Solaris, but not Windows?!)

No, those were programmed exclusively for POSIX OSes. If you're searching for an open-source paint program intermediate in its complexity, try Pinta: https://pinta-project.com/pintaproject/pinta/

I have never heard of this game before, and have just checked out the link you provided. It's quite interesting.