Since Microsoft no longer hosts Windows 7 downloads on their primary consumer sites, you have two reliable options for obtaining an image:
if you are running a massive topology and need to save on RAM. It typically requires only 1GB to function smoothly. download windows 7 for gns3
Here is everything you need to know about finding and setting up a Windows 7 instance for your GNS3 topology. Finding a Legitimate Windows 7 ISO Since Microsoft no longer hosts Windows 7 downloads
Many users have uploaded original, untouched ISOs here. Look for "Windows 7 Professional" or "Ultimate" with high view counts and positive comments to ensure the file hasn't been tampered with. download windows 7 for gns3