Each account, both free and paid, has an upload limit based on file size. This is calculated from the spreads uploaded, images attached to comments and reviews, and your specified watermark.
Spreads imported from older revisions and left unchanged will not be double counted.
Once the limit is hit you will no longer be able to upload files. This also means that your clients will be unable to attach images while reviewing their albums.
To get more space you can either upgrade to a paid plan or delete older albums.