Will one mongoose collection slow down another?
I understand that this is not a direct answer to your question, however IMHO if you're going to do something, do it right the first time and save your images into something like S3.
S3 has a free tier that you can use for 1 year. It will give you 5 gbs of space, 20 000 GET requests and 2000 uploads per month (honestly if you're going over that you can probably afford to pay for a small plan). You can read about AWS's free offerings here.