@Wathik This limit is intentional. You can read more about account limits here. If you want to store more than 450 images, we do not currently offer a solution other than downloading backup copies and deleting the excess images. In the future, we may offer a paid option that provides more storage than the current 450.
If you search the forum, you will find that many members have built scripts to perform the task of local image backup. Here is an example that I wrote some months ago.