Differential backup same size as of Full Backup
This was originally left in a comment by Dan Guzman:


  The differential backup includes all extents modified since the last full backup. If you reorganize or rebuild after the full backup, the diff backup will include all those modified extents. Note when you reorg with LOB_COMPACTION ON, the differential backup size will increase significantly if LOB columns are a large portion of the database space.


If you've performed a big maintenance action (rebuild, or LOB compaction) on the tables with images in them, you'll likely see the differential approach or reach the size of the full database backup, as is documented on the MS Docs page Differential Backups (SQL Server):


  A differential backup captures the state of any extents (collections of eight physically contiguous pages) that have changed between when the differential base was created and the when differential backup is created. This means that the size of a given differential backup depends on the amount of data that has changed since the base.


Another possibility, mentioned by Midhun C N, is that the "differential base" (the last full backup) is very old.

As far as actions you can take:


verify that full backups are completing successfully, so that the differential base is getting updated
consider avoiding large rebuilds or reorgs on the tables with image data in them unless there is a compelling reason to do so
if maintenance actions are necessary, try to space them out across multiple full backup iterations, to limit the size increase of differentials
if splitting them up is not possible, make sure to plan for the increased size of the differentials