Paul segues from thanking the Father (1:11b-14) to exalting the Son (1:15-20). This is the One we are to honor and serve as Lord (1:10) and King (1:13). Thus, Paul develops for us a high view of Christ as supreme over all things so that we would worship and serve Him for the exalted Son of God that He is.
Image (15a): Paul first identifies Christ in His incarnation as "the image of the invisible God." The incarnate Son is the representation of the invisible God in visible humanity. In His earthly existence, Jesus revealed the Father through His attributes (John 1:14), miracles (John 2:11; 11:40) and words (John 14:10, 24; 17:8). Thus, the Son explained the Father (John 1:18), and He even boldly asserted: "He who has seen Me has seen the Father" (John 14:9).
Firstborn (15b-16): Moreover, He's "the firstborn of all creation." In terms of chronology, the incarnation isn't the first. Christ entered the world long after creation. In terms of status, however, He is the "firstborn," pre-eminent over all things. This exalted status is explained in 1:16, "For by Him all things were created." He is supreme over all because He created them all. All things "in the heavens and on earth, visible and invisible" (even angels!) have been created through Him, and for His sake! This is His world, and He is supreme over all!
 1:15 continues the sentence with a relative pronoun ("who [hos (ὅς)]"), not with "He" (NASB).
 The apposition "the firstborn of all creation" explains "the image" as Christ's human existence in creation.
 "Image [eikōn (εἰκών)]" means a representation or an impression of something in another form, an image.
 Scripture attests to God's invisibility. Cf. Ex. 33:20; John 1:18; 1 Tim. 1:17; 6:16; Heb. 11:27; 1 John 4:12.
 This display of incarnate glory should not be confused with His eternal glory, set aside at His incarnation and restored to Him at His glorification (John 17:5, 24). With this glory He will return (Matt. 16:27; 25:31).
 This word can describe status, not birth order, hence, pre-eminent. This fits His post-creation humanity best.
 The phrase "thrones or dominions…" explain the invisible things. These are angels (cf. Eph. 6:12).