What Backup Compression Rate Can I Expect for a Database with Mostly JPG Data?
I finally went ahead and got a test database loaded with data. Table structure is:

CREATE TABLE [dbo].[ImageTable](
    [CompositeID] [bigint] NOT NULL,
    [ImageID] [tinyint] NOT NULL,
    [FullImage] [varbinary](max) NULL,
    [ThumbnailImage] [varbinary](max) NULL,
    CONSTRAINT [XPK_ImageTable] PRIMARY KEY CLUSTERED ([CompositeID] ASC,[ImageID] ASC)
)


The table was loaded with approximately 2.8M records. The FullImage value was set, and the ThumbnailImage value was left as NULL for each record.

The database itself is 154GB in size.
The uncompressed backup is 154GB in size.
The compressed backup is 134GB in size.

Given that the two PK fields account for approximately 24GB (2.8M records * 9 bytes/record), it looks like the only savings from compression are being realized by compressing the PK fields.