John,
It can be challenging! I try to always, when I write a caption, copy and paste it to the file info description. Often, I'll copy a portion of a post or other info into the description, in quotes. If you have to try copying captions from various sources that you already have, it only takes a moment to add it to the file info, then you have it with the image, permanently as a back-up. Anyplace you post the image, like here, will then be a back-up of the info that you can easily access. Many programs can access the file info, so others in the future can also look to see if there is more detail than what is in the published caption.
Although the description box is small, way more text can be placed here than the size of the box. Sometimes an answer to a question about an image location or other trivia will be posted and I usually try to add that info and a note of where it came from. Over time it gets to be a habit. There's no doubt, this has saved me on several occasions when I can't remember some little detail.
Somewhere I heard or read that
the task that never gets started is the one that takes the longest!
Have fun!
Steve