Is it possible to reduce the size of an image imported into Socratic via its URL?
This should be possible. I'm not exactly certain if you are trying to reduce the size stored on Socratic or just get a reduced size for your local use.
If you simply want to retrieve an image that has been imported into Socratic and reduce its size locally, there should be no problem doing this. If you want to change its size on Socratic you might need to modify it locally then re-edit the answer to use your uploaded and modified version.
Assuming you are working in Windows (other OS's may need other methods) simply left click on the image, then open your favorite image processing program (e.g. MS-Paint), resize and resave.