In /Applications/Utilities you will find "Bluetooth File Exchange". You can use this to send files to your phone. Be advised though that many phone models don't give you a warning message if they don't have enough memory to accept the file, they just don't even try it, so if you can't send a file and can't work out why, check you have enough space on your phone.
To send files to your Mac from your phone, find the file you want (eg a photo) and choose Send -> Via Bluetooth. Your iMac should appear in the list. Depending on how you've setup the pairing in Bluetooth in System Preferences, you may be prompted to accept the file on the Mac.