It is quite surprising that Ubuntu does not ship with a decent replacement for Microsoft Paint by default. This is a shame since pasting screenshots, marking out rectangles and such are basic operations you may need to do on images.
There are fully features alternatives to MS Paint, like Pinta, but they are quite heavy and need installing many packages. I like GNOME Paint, which is an exact clone of MS Paint. You get the exact same interface and features and nothing more.
Install it like this:
$ sudo apt install gnome-paint
Tried with: GNOME Paint 0.4.0 and Ubuntu 14.04