[Misc] Support local image encoding in benchmarks (#43843)

Signed-off-by: xiaoz <Sukra1@outlook.com>

docs/benchmarking/cli.md

@@ -246,6 +246,12 @@ Every image listed in "image_files" is added to the request in the listed order
 
 The "image" shorthand accepts the same values as "image_files". The "image_url" field accepts either an OpenAI-style object with a "url" field or a URL string.
 
+By default, image references are sent to the serving endpoint as provided, with local image paths converted to `file://` URLs.
+
+If the benchmark client should load local and HTTP(S) images before sending requests, pass `--custom-ensure-client-side-data` to encode them as base64 data URLs on the client side.
+
+Existing `data:image/...` URLs are already self-contained and are kept unchanged.
+
 ```bash
 # need a model with vision capability here
 vllm serve Qwen/Qwen2-VL-7B-Instruct
@@ -253,13 +259,13 @@ vllm serve Qwen/Qwen2-VL-7B-Instruct
 
 ```bash
 # run benchmarking script
-vllm bench serve--save-result --save-detailed \
+vllm bench serve --save-result --save-detailed \
   --backend openai-chat \
   --model Qwen/Qwen2-VL-7B-Instruct \
   --endpoint /v1/chat/completions \
   --dataset-name custom_image \
   --dataset-path <path-to-your-image-data-jsonl> \
-  --allowed-local-media-path /path/to/image/folder
+  --custom-ensure-client-side-data
 ```
 
 Note that we need to use the `openai-chat` backend and `/v1/chat/completions` endpoint for multimodal inputs.