Imlib2 is the (intended) successor to Imlib.
 
Imlib2 can load image files from disk in one of many formats, save
images to disk in one of many formats, render image data onto other 
images, render images to an X-Windows drawable, produce pixmaps and
pixmap masks of images, apply filters to images, rotate images,
accept RGBA data for images, scale images, and more.

Slackware comes with Imlib 1.9.x - Note that the imlib2 site says:
"Imlib2 can be installed alongside Imlib 1.x without any problems since 
they are effectively different libraries - but they have very similar 
functionality."