Launch the application Preview - it's in your Applications Folder.
Use it to open the pics in question - drag them to the Preview icon in the dock, or use the File -> Open command. File -> Open means the Open... command in the File Menu)
When they are open you can use the Save As command - that's in the File menu too - like on every other Mac application.
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how to convert bmp to jpeg or gif on mac please ?
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