There is no need to use third-party software to crop images, as Miro has a tool for it. Double click on the image or choose Crop icon in the context menu.You can always revert your cropped image to its original, just double click and expand the crop selection to its original size.
Crop icon in the context menu
This enables the Crop mode, which allows editing of the rectangular selection. When you are finished, just click anywhere on the board.