From single site to multi site of an existing site broke all pages images
I reckon that the paths which are stored in the database refer to the old url. In such case, you need to run a SQL command inside MySQL that corrects the paths.

To fix entries inside field_data_body table, you can run this command:

UPDATE field_data_body SET body_value = REPLACE(body_value, 'sites/all/files', 'sites/mysite.com/files');


You have to do the same for the corresponding revision table.