Consideration for a little bit heavy database
This table should have a clustered index on UserId. If there is no more than one thumbnail per user, then the clustered index can be the primary key.

Since you say you have a currently-running system on SQLServer 2012 standard edition, online index builds are not possible. You could wait for a low-activity period to create the index, or create a new table, migrate the data into it, then switch to using the new table.

The table should ideally look like this:

CREATE TABLE RegisteredThumbnail
(
    UserId       INT            NOT NULL,
    ThumbData    VARBINARY(MAX) NOT NULL,
    Created      DATETIME2(7)   NOT NULL,
    Updated      DATETIME2(7)   NULL,

    CONSTRAINT PK_RegisteredThumbnail PRIMARY KEY CLUSTERED (UserId ASC)
)


Since you cannot recreate the table, this is the index you need to create:

CREATE INDEX IX_RegisteredThumbnail_UserId
ON RegisteredThumbnail(UserID ASC)


As you're on 2012 Standard, this will be an "offline" operation - it'll block activity on this table while the index is being built. Given it's only a million rows, it should not take too long to build, however it does have to scan the table in order to do it, so you'll be limited by how fast it can read that data.