Here is the source URL of the image -

https://erp.guu-portal.de/erp/dokumente/download/759927?filename=9783835408241.jpg

curl result for the image file -

curl -v 'https://erp.guu-portal.de/erp/dokumente/download/759927?filename=9783835408241.jpg'

*   Trying 62.245.203.125:443...
* Connected to erp.guu-portal.de (62.245.203.125) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
* successfully set certificate verify locations:
*  CAfile: /etc/ssl/cert.pem
*  CApath: none
* TLSv1.2 (OUT), TLS handshake, Client hello (1):
* TLSv1.2 (IN), TLS handshake, Server hello (2):
* TLSv1.2 (IN), TLS handshake, Certificate (11):
* TLSv1.2 (IN), TLS handshake, Server key exchange (12):
* TLSv1.2 (IN), TLS handshake, Server finished (14):
* TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
* TLSv1.2 (OUT), TLS change cipher, Change cipher spec (1):
* TLSv1.2 (OUT), TLS handshake, Finished (20):
* TLSv1.2 (IN), TLS change cipher, Change cipher spec (1):
* TLSv1.2 (IN), TLS handshake, Finished (20):
* SSL connection using TLSv1.2 / ECDHE-RSA-AES256-GCM-SHA384
* ALPN, server accepted to use http/1.1
* Server certificate:
*  subject: CN=*.guu-portal.de
*  start date: Jan  5 00:00:00 2021 GMT
*  expire date: Feb  5 23:59:59 2022 GMT
*  subjectAltName: host "erp.guu-portal.de" matched cert's "*.guu-portal.de"
*  issuer: C=GB; ST=Greater Manchester; L=Salford; O=Sectigo Limited; CN=Sectigo RSA Domain Validation Secure Server CA
*  SSL certificate verify ok.
> GET /erp/dokumente/download/759927?filename=9783835408241.jpg HTTP/1.1
> Host: erp.guu-portal.de
> User-Agent: curl/7.77.0
> Accept: */*
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Date: Fri, 14 Jan 2022 17:55:16 GMT
< Server: Apache-Coyote/1.1
< X-Content-Type-Options: nosniff
< X-XSS-Protection: 1; mode=block
< Cache-Control: no-cache, no-store, max-age=0, must-revalidate
< Pragma: no-cache
< Expires: 0
< X-Frame-Options: SAMEORIGIN
< Last-Modified: Thu, 18 Oct 2018 20:32:21 GMT
< Content-Disposition: attachment; filename=9783835408241.jpg
< Content-Type: image/jpeg;charset=UTF-8
< Content-Length: 182163
< Set-Cookie: JSESSIONID=30BDB73806080AEF778934ABEBADFFCE.worker1; Path=/erp; HttpOnly
< 
Warning: Binary output can mess up your terminal. Use "--output -" to tell 
Warning: curl to output it to your terminal anyway, or consider "--output 
Warning: <FILE>" to save to a file.
* Failure writing output to destination
* Closing connection 0
* TLSv1.2 (OUT), TLS alert, close notify (256):

I was sending payload in post request -

payload = {'image': {'src': 'https://erp.guu-portal.de/erp/dokumente/download/759927?filename=9783835408241.jpg', 'filename': '399_1.jpg'}}

Posting images to rest API URL -

requests.session.put(base_url + f"products/{product_id}.json",payload))

Response:

Status Code 422 with Message: {"errors":{"image":["Could not process image: [\"\/erp\/dokumente\/download\/759927 (image\/jpeg;charset=UTF-8) is not a recognized format\"]"]}}

I have tried other public images and they work... however, they had content-type image/jpeg only