As you may have read by now, the roadmap to HTTPS will involve some manual fixing of dead links to images and uploading images from other sources to imgur. Sooner or later this is going to come our way, so let's get cracking now. The SE devs have a list of posts for MSE, but for the rest of us, this SEDE query can be used to keep track of posts (unless we can come up with better ideas).
There would be thousands of such posts, so we would need to pace ourselves to avoid burnout and flooding the active questions list.
For non-imgur non-HTTPS links, the guidance is to upload to imgur where possible:
The "upload to imgur" link opens the image upload dialog with the link pre-populated.