To upload an image for a tag, use the command resource/upload_tag_image:

svc=resource/upload_tag_image&params={"itemId":<long>,
				      "tagId":<long>,
				      "eventHash":<text>}

 To delete an image, use the command Tags (e.g. passengers): create, edit, delete.

To upload an image, use a POST request with multiple contents (multipart/form-data), where one part contains parameters and the other contains the image.

For example:

Request URL: https://hst-api.example.com/example/ajax.html?svc=resource/upload_tag_image&sid=8157df114c0e601f0f31091c3c2ac53d
Request Method: POST
Host: hst-api.example.com
Connection: keep-alive
Content-Length: 31755
Cache-Control: no-cache
Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryB4wasXYYHLTNXHBl
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Charset: windows-1251,utf-8;q=0.7,*;q=0.3
Accept-Encoding: gzip,deflate,sdch
Accept-Language: ru,en-US;q=0.8,en;q=0.6

------WebKitFormBoundaryB4wasXYYHLTNXHBl
Content-Disposition: form-data; name="params"

{"itemId":717314,"tagId":17,"eventHash":"jUploadForm1372768554999"}
------WebKitFormBoundaryB4wasXYYHLTNXHBl
Content-Disposition: form-data; name="eventHash"

jUploadForm1372768554999
------WebKitFormBoundaryB4wasXYYHLTNXHBl
Content-Disposition: form-data; name="tags_dlg_props_upload_image"; filename="image.jpg"
Content-Type: image/jpeg

------WebKitFormBoundaryB4wasXYYHLTNXHBl--

{}	/* empty object if execution successful, if not - error code */

To be sure that the image has been uploaded, you can execute the command Events:

{
	"tm":<uint>,				/* current server time (UTC) */
	"events":[
		{
			"i":<long>,			/* tag ID (-1 if unknown) */
			"d":{				/* data */
				"hash":<text>		/* upload is complete */
			}
		}
	]
}