Below is a sample that calls this UDF with chunks of 2400 characters (because of the way Base64 encoding works, the “chunk” size has to be divisible by 4).

CREATE PROCEDURE testConvert
    @someParameter int,
    @attachmentData text
AS

/*** Table schema used for test
CREATE TABLE testData(someValue int, attachmentData image, CONSTRAINT PK_testData primary key nonclustered (someValue))
***/

-- insert NULL (0x0) into the image field so that the TEXTPTR function will work
insert testData(someValue, attachmentData)
values(@someParameter, 0x0)

declare @pointer varbinary(16)
select @pointer = TEXTPTR(attachmentData) from testData where someValue = @someParameter

declare @buff varchar(2400)
declare @offset int, @imgOffset int
set @offset = 1
set @imgOffset = 0
while @offset <= datalength(@attachmentData)
begin
 select @buff = substring(@attachmentData, @offset, 2400)
 declare @img varbinary(8000)
 select @img = dbo.base64toBin(@buff)
 UPDATETEXT testData.attachmentData @pointer @imgOffset NULL @img
 set @imgOffset = @imgOffset + datalength(@img)
 set @offset = @offset + 2400
end