gpui: Make image example work regardless of how it is run (#24645)

This PR updates the GPUI `image` example such that it works when run in
the following ways:

- `cargo run -p gpui --example image` from the repository root
- `cargo run --example image` from within `crates/gpui`

Release Notes:

- N/A

Cargo.lock

@@ -5546,6 +5546,7 @@ dependencies = [
  "rand 0.8.5",
  "raw-window-handle",
  "refineable",
+ "reqwest_client",
  "resvg",
  "schemars",
  "seahash",