Remember that you look AT, not 'to': Let's have a look at that picture. Let's take a look at that picture. Both of the above sentences are correct. We have many phrases in English with 'verb + a + noun' ( for example 'have a shower'/ 'take a shower') where you can use either 'take' or 'have'. As a general rule, phrases with 'take' tend to be used more in US English, while phrases with 'have' tend to be used more in British English. The choice is yours!

You "look at" something. "Let's have a look at that picture" and "Let's take a look at that picture" have the same meaning. Remember that come common words (such as have and take) can be very flexible. The best tactic is to simply accept that's what people say.