In Windows 7 you do not have the transparency feature in Paint. This is by design. However, you may post your suggestion or feedback on this incase you would like to see this implemented in the future.
However, you may search Internet using your favorite search engine to check if you can download any free program such as paint to edit the pictures which includes transparency feature.
Note: Using Third Party Software, including hardware drivers can cause serious problems that may prevent your computer from booting properly. Microsoft cannot guarantee that any problems resulting from the use of Third Party Software can be solved. Using Third
Party Software is at your own risk.