There are 3 ways to flash a recovery, depending on what type it is.
1. If it is a recovery.zip type, just flash it in recovery like any other zip file.
2. If it is a .tar.md5 or .tar file, flash it in Odin.
3. If it is a img file, rename it to recovery.img, and copy it to the root of your SD card, in terminal or adb shell, do this command:
dd if=/sdcard/recovery.img of=/dev/block/mmcblk0p2
If you want to flash as img but the file is zip or .tar.md5, simply open up the zip file and extract the img file inside. For .tar.md5, rename it to .tar first, and extract it. The img file is inside too.
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