One easiest solution is to go under Picture Tools Format tab and click on “Picture Effects”, which is located in the Picture Styles section. From there choose “Preset” and move to “Preset 8”. Simply hover over to see the live preview and if it suits you.

Depending on your picture another tool under the same tab in Adjust section may also work for you. Click on “Recolor” and choose “Set Transparent Color” from the menu. Click with the new cursor on the color which should show transparent and see if this works for you.