Exporting to PNG
You can export your wireframes to image files by selecting the Project menu > Export... option (File menu > Export... in Balsamiq for Desktop), or the dedicated keyboard shortcut CTRL/⌘+E, and then the PNG tab from the dialog.
From there, you can generate standard PNG files or high resolution ones, depending on your needs.
You can choose to export all wireframes, the current one or a specific set, including any alternates you have created.
Note: If you check the "Use Transparent Background" option, the exported image(s) will be transparent anywhere the canvas shows through in the wireframe.
You can also export a subset of the controls in your wireframe by selecting them on the canvas and using the Export Current wireframe option. The export dialog will ask you if you want to export the entire wireframe or only the selected controls.
Note: In Balsamiq for Desktop, you can also export your images as individual PNG files if needed.