Reset iMac without losing photos.

You have provided so little information, we cannot possibly know what you'll lose. Here are some examples:

• Where are these photos housed? Are they only on your Mac? Do you keep them, for example, in iCloud Photos?
• You do back up your Mac using MacOS' Time Machine, right? If so, the photos are very likely saved on this backup (depending in the answer to the bullet question above).

Back them up before you erase the disk. Otherwise, you will lose everything when the disk is erased.