welcome Bernardo to this forums, I’m sure you will learn a lot here, for your snow matte painting, i think you have to work more on how the different photo elements should match in one single image, first of all, the lighting direction ,shadow casting and color temperature, the shadow and highlight colors are very important, I can see some elements with blueish highlights/snow and some other with totally white non saturated snow, they all should match in this values, where you have your lens flare now in not where the real sun would be, according to your shadows the sun should be probably off to the left , for your character integration you need to work more, thinking about values in brightness and darkness , as well as shadow projection, and color
the jungle one is kind of confusing to me, you need to work under linear perspective rules, for distance, and proper scale , you have a nice idea of mood and color palette you just need to push it more into perspective, and more detail, again the lighting is confusing
in the paradise one , linear perspective is needed as well, some of the elements are totally off reality , and for this one aerial perspective will help you a lot with sense of scale and distance
browse the tutorial section inside this matte painting forum and you will understand all the rules and techniques behind this art
matte painting is a serious art and needs a lot of disciplines be patient, don’t run before walking , and little by little you will get better and your eye will start to understand what you need to create better artwork, good luck and keep it up !