I have read this somewhere '...then Al-Jabbar will come to them in an image different than the image in which they saw Him the first time...' (Al-Bukhaari 7440, Muslim 183). Does this hadith mean that Allah, The Exalted, is capable of changing his image?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

Allah Almighty is capable of everything, and nothing is difficult for Him. It is sufficient for you to believe in what the hadeeth indicates; that Allah comes to them in another image, without trying to investigate that.

The hadeeth which you mentioned is explicit that Allah comes to His faithful worshipers on the Day of Resurrection and appears to them in an image other than the image which they saw Him in previously as a test for them. He will say (to them), “I am your Lord.” They will say, “We take refuge with Allah from you. We will stay here till our Lord comes to us.” And when our Lord comes, we will recognize Him. Subsequently, Allah will come to them in His own image, recognizable to them, and say, “I am your Lord.” They will say, “You are our Lord…” 

This hadeeth was narrated with different wordings.

Shaykh Al-Ghunaymaan mentioned in Sharh Kitaab At-Tawheed from Saheeh Al-Bukhaari that it was proven that they will see Allah several times in several images, and this is a test for them, as proven by the saying of the Prophet “Al-Jabbaar (the Restorer and Formidable – i.e. Allah) will then come to them in an image other than His own image in which they saw Him the first time…” [Al-Bukhaari and Muslim]; and his saying: “Then (Allah) the Lord of the worlds will come to them in an image nearest to the image they had seen Him in the first time,” as in the narration by Muslim. Also, the Sunnah of Ibn Abu ‘Aasim reads, “Then Allah appears to us in an image other than His image in which we first saw Him.” In another narration by him also: “Then he raises our righteous [accepts their deeds] and our sinners [pardons them], while He has turned into the image we saw him in the first time.”

The hadeeth by Muslim reads, “…then they raise their heads and see that He has turned back to the image which they saw Him in the first time.” He will say, “I am your Lord.” They will say, “You are Our Lord…”

The wordings of these ahaadeeth are explicit in that they will see Him in the image in which they knew Him before, before he comes to them this time.

Allah knows best.