Откуда эта скорость передачи?

A 56k modem would virtually never provide a 56k connection due to a whole raft of issues - including impedence mismatch and other cabling faults and hardware incompatibilities. 40-48k seemed to be the norm from memory - connections over 53k were very, very rare.

Also, most modems followed the "V90" standard which was asymetrical and limited upload speeds to 33.6k - not 48k of the "V92" standard.

[ Source: I used to run an ISP in the days of dialup and invested quite heavily in special 56k capable modems - for a 56k connection the ISP could not use a standard modem they needed to use a 64k ISDN channel ].

Also, this whole post is simplistic (you could argue flawed if you want to be pedantic) - That 50kb * 8kbits is naive as it ignores the overheads of sending the file - this is quite significant on an Internet connection but even on a basic connection it was non-trivial - think an overhead of 10%

As someone else has posted, if the image is being sent in an email, there is substantial overhead in converting the file from 8 bits of ASCII per character to something which falls in the acceptable space for email (readable characters, line limits). This could vary by encoding method - there is more then 1, but in the region of 33% overhead.

I wonder where "40,000 bits/sec" comes from in the transmission calculation?

A 56.6 Kbps modem connection uses V.92.

The upload speed is a max of 48 kbit/s, but at the expense of download rates.

A 48 kbit/s upstream rate would reduce the downstream as low as 40 kbit/s due to echo on the telephone line.

Using digital lines and PCM (V.90/92)

The International Telecommunication Union (ITU) announced the draft of a new 56 kbit/s standard, V.90, in February 1998 with strong industry support.[4] Incompatible with either existing standard,[4] it was an amalgam of both which was designed to allow both types of modem to be converted to it by a firmware upgrade. This V.90 standard was approved in September 1998, and widely adopted by ISPs and consumers.[4] Later in V.92, the digital PCM technique was applied to increase the upload speed to a maximum of 48 kbit/s, but at the expense of download rates. A 48 kbit/s upstream rate would reduce the downstream as low as 40 kbit/s due to echo on the telephone line.

Source Modem

Normally, the 56600 bps rate should be used in this calculation. However, e-mail attachments typically use base64 encoding where each 3 8-bit bytes are encoded by 4 7-bit ASCII characters, and most e-mail clients can only handle 8-bit characters anyway, so the net result is transforming 3 bytes into 4, and the effective transfer rate will be

56600 / 4 * 3 = 42450 (bps) 

PS. As @David's answer points out, a 56.6k modem has its upload speed limited at 48000 bps. Applying the same "correction" for this rate gives

48000/ 4 * 3 = 36000 (bps) 

Still, none of these calculations give the exact 40000 bps result, so I suppose it's an approximative value.