"..image of the invisible God, the firstborn of all creation" tells us that God is invisible (Jesus was visible) and that Jesus is the "firstborn", God is eternally preexisting. Therefore this verse does not support that Jesus is God, but an "image" of God. An image of anything be it God, a human being, or an inanimate object does not have the innate nature of what it represents and we are told in Hebrews 1:3 that Jesus is the exact representation of God's nature.
Now, let's look at the Greek translated "firstborn". "Prototokos" means the "first to open the womb". In Collossians 1:18 Paul tells us that Jesus is the "prototokos" of the dead. Now when we put vs. 15 in context we can understand that Jesus is the "prototokos" of the new creation who was given all authority over thrones, dominions, rulers, and authorities (Matthew 28:18).
To interpret Colossians 1:15-16 to mean Jesus is co-equal and co-eternal with the Father is to propose that Paul contradicts himself since in 1 Corinthians Paul wrote:
1st Corinthians 8:5-6
5 For even if there are so-called gods whether in heaven or on earth, as indeed there are many gods and many lords, 6 yet for us there is but one God, the Father, from whom are all things, and we exist for Him; and one Lord, Jesus Christ, by whom are all things, and we exist through Him. NAS
This interpretation also contradicts Jesus' own words in John 17:
John 17:3 "And this is eternal life, that they may know Thee, the only true God, and Jesus Christ whom Thou hast sent. NAS
The Greek word for "only" is "monon":
NT:3441
monos (mon'-os); probably from NT:3306; remaining, i.e. sole or single; by implication mere:
KJV - alone, only, by themselves.
(Biblesoft's New Exhaustive Strong's Numbers and Concordance with Expanded Greek-Hebrew Dictionary. Copyright (c) 1994, Biblesoft and International Bible Translators, Inc.)
NT:3441
monos, monee, monon,
1. an adjective, alone
a. with verbs: Matthew 14:23
b. it is joined with its noun to other verbs also, so that what is predicated may be declared to apply to some one person alone Matthew 4:10
2. Neuter monon as an adverb, alone, only, merely: Matthew 5:47
(from Thayer's Greek Lexicon, Electronic Database. Copyright (c) 2000 by Biblesoft)