Use this

https://image.tmdb.org/t/p/original/[poster_path]

https://image.tmdb.org/t/p/original/wigZBAmNrIhxp2FNGOROUAeHvdh.jpg
Make sure to read the documentation around images.

See here: https://developers.themoviedb.org/3/configuration/get-api-configuration
Some older ones may still work. But they can die at any time.
These are official sizes:

images  
base_url    "http://image.tmdb.org/t/p/"
secure_base_url "https://image.tmdb.org/t/p/"
backdrop_sizes  
0   "w300"
1   "w780"
2   "w1280"
3   "original"
logo_sizes  
0   "w45"
1   "w92"
2   "w154"
3   "w185"
4   "w300"
5   "w500"
6   "original"
poster_sizes    
0   "w92"
1   "w154"
2   "w185"
3   "w342"
4   "w500"
5   "w780"
6   "original"
profile_sizes   
0   "w45"
1   "w185"
2   "h632"
3   "original"
still_sizes 
0   "w92"
1   "w185"
2   "w300"
3   "original"

As for the import error, these are the error messages from the TMDb servers.
See here https://www.themoviedb.org/documentation/api/status-codes
If it is not one of these messages, it is probably a message from another service.

See here
https://developers.themoviedb.org/3/configuration/get-api-configuration

Upload - Supported Image Sizes (in pixels)

| API name |      Item       |   Min Res  |   Max Res   |
| :------: | :-------------: | :--------: | :---------: |
|  poster  |     Poster      |  500 x 750 | 2000 x 3000 |
| backdrop |     Fanart      | 1280 x 720 | 3840 x 2160 |
|   still  | TV Show Episode | 1280 x 720 | 3840 x 2160 |
|  profile |    Cast/Crew    |  300 x 450 | 2000 x 3000 |

API - Supported Image Sizes

|  poster  | backdrop |  still   | profile  |   logo   |
| :------: | :------: | :------: | :------: | :------: |
| -------- | -------- | -------- |    w45   |    w45   |
|    w92   | -------- |    w92   | -------- |    w92   |
|   w154   | -------- | -------- | -------- |   w154   |
|   w185   | -------- |   w185   |   w185   |   w185   |
| -------- |   w300   |   w300   | -------- |   w300   |
|   w342   | -------- | -------- | -------- | -------- |
|   w500   | -------- | -------- | -------- |   w500   |
| -------- | -------- | -------- |   h632   | -------- |
|   w780   |   w780   | -------- | -------- | -------- |
| -------- |  w1280   | -------- | -------- | -------- |
| original | original | original | original | original |  

Original Size is the size of the uploaded image.
It can be between Min Res (Minimum Resolution) and Max Res (Maximum Resolution).