Sure, but compared to the stickers UI paradigm it's far clunkier to browse for an image file every time and to collect and arrange image files sent by other people, particularly on a mobile device.

Also, stickers display with proper transparency, giving them the appearance of arbitrary shapes, whereas images on Signal are displayed in a rectangular frame and transparent areas get a black background.

Compare:

https://image.ibb.co/niKWUQ/IMG_20170926_135611.jpg

with:

https://preview.ibb.co/fg21UQ/IMG_20170926_135627.jpg

It may seem trivial, but this matters to people in the real world.