Okay. First note: never save pixel art as .jpg. It adds random pixels and generally messes up the artwork. I would reccommend .png as a format to save in.
Secondly, on to the shed. Below is a quick sketch to demonstrate how to draw perspective-correct roofs in isometric art.
-As you can see, I started with your shed.
-First of all, I removed the colors and the .jpg artifacts from the white space.
-Then I reduced the lines to a box.
-In the next step, I drew lines connecting the corners of the face of the box above which the gable of the roof was to be placed. These lines are not quite necessary in standard 2:1 iso, but they are helpful if you are drawing in different perspectives. I extended a line up from the center of the "x", which corresponded to the center of the gable and thus the peak of the roof. I drew lines from the end of the fuscia line to define the roofline.
-In the next drawing, I removed the fuscia guidelines and drew a similar gable on the other end and a ridgeline to connect the two.
-In the sixth drawing, I cleaned up the extra lines.
-I added some colors in the final drawing.
I hope this is helpful, and if you have any questions about what I did, feel free to ask.