A system image is just another file as far as the external hard drive is concerned. If you use the native Windows 7 imaging tool, the system image file it creates on the external hard drive will be called WindowsImageBackup. Change the name to something unique like My Computer. When your brother does his image you'll see two files: My Computer (the system image you created) and WindowsImageBackup (the image created by your brother.) I'd suggest renaming his image to something unique like His Computer. That way there won't be any doubt which image belongs to which computer. If you ever have to use your image, rename it back to WindowsImageBackup. Same for your brother.
If you use a different imaging program like Macrium Reflect free, it will assign a different file number to each image. But I'd suggest you rename each file by adding a "mine' or "his" to the file number just to keep it straight which image belongs to who. If you have to use the image, just delete the "mine" or "his" from the unique number.