media card的图片怎么设置大小？

#### 实现场景：
crud的card模式展示海报图，用多媒体card模式，由于海报图大小不一致，希望把缩略图统一设置成固定的240*300大小。

#### 存在的问题：
media card无法设置图片大小，所有图片全部原始大小展示，加上className也无效

#### 当前方案：
请粘贴你当前方案的完整 amis schema 代码...

```
"card": {
"className": "text-center",
"header": {
"title": "$title",
"subTitle": "$intro"

},
"media": {
"type": "image",
"className": "inline m-36", //设置inline是为了居中显示图片，实测inline可以覆盖原来的block属性达到居中显示效果。但width属性无法覆盖，F12看到的图片的属性仍然是auto展示原始大小。
"url": "$url",
"position": "top"
}
}
```

Start with the provided card.media schema and trace how the media card renders its image and applies className. Confirm how a fixed 240×300 thumbnail should be configured, then verify that differently sized poster images render consistently in the card mode.

Written by the indexing model from the issue text.