Remember the importance of being natural. However, you should use sentences such as the following ones in order to accomplish your task in a better way:
-To be honest with you,
-I am of the opinion that…
-I strongly believe that…
-If I had to make up my mind, I would say that…
-I guess that…
When you are commenting on the pictures you can say something like
-Both pictures show that… In the first picture I can see… whereas in the second one he/she appears to be + verb-ing
-The main similarity that I can appreciate is that/ the principal differecence is that…
-About the quetion, I have to say that…
For the dialogue you can use questions such as
- Do yo know what I mean?
– Do you see my point?
– What is your view about my idea?
– Shall we move on to the next picture?
– What would you say about this picture over here?
– Don´t you think so?
To agree and disagree you could use
- I agree with you up to a point because
– I see your point, however, I am of the opinion that…
– I know what you mean. Although if I have to be honest, I would never choose that option as I consider the other ones to be more important.
– I 100 per cent/ totally/ agree with you.
– No, I don´t see the point in choosing that picture.
- Despite all that you´ve said, I guess that I cannot completely agree with you, as I believe that…
– Oh!!! that is a good idea, though, you should take into account that the others pictures portray the idea better.
To reach an agreement you may want to say something like
- Okey as we have to make up our minds I would say that these two over here are by far the best two options. What do you reckon?
– Yeah, regarding everything we´ve said I think you are totally right.