here's my internet power user opinion:

a malicious image, as in that opening it in a vulnerable program (browser or image viewer) would allow code execution?

those exploits usually require specially crafted image files, so i think re-encoding and optimizing the image (like using `jpegtran -optimize -copy none` or `optipng`) would possibly do for "sanitizing" the file

a code execution flaw in jpegtran or optipng would be worse though, but sandboxing might work to protect against them (i don't know much about sandboxing though)