Sage comme une image is an expression used to talk about somebody's - usually a child's - calm behavior. We often translate sage by good. It also means quiet. Une image is a picture. Whatever is on the picture is immobile, quiet. Sage comme une image is literally Good/quiet as a picture. Quiet as a mouse is an equivalent expression in English. A mom coming back from an appointment, asks the nanny about her daughter: Mom: 'Elle a été sage aujourd'hui?' Nanny: "Oh oui! ...comme

ça va barder! is used when an argument is about to take place. An equivalent in English is Things are going to get ugly. Let's say you're talking to a colleague who hasn't completed the project she should have completed last month: " T'as pas encore fini ton projet? ça va barder, tu vas voir!" (meaning you're going to hear it from the boss!)

Faire une croix sur... is an expression used to indicate you are renouncing something, you are saying goodbye to something. For example, your brother just learned he has diabetes. You can tell him: "Ben, tu peux faire une croix sur les gâteaux!" (meaning no more eating cakes)