Previously, there was no option to specify the model for image generation in the OpenAI configuration. This commit adds a new option called "ImageModel" where you can specify the desired model. The default value for this option is "dall-e-2".
In the `GPTBot` class, the OpenAI object is now initialized with the `ImageModel` option if it is provided in the configuration. This allows the bot to use the specified image generation model in addition to the chat model.
Furthermore, in the `OpenAI` class, the `image_api` attribute has been renamed to `image_model` to reflect its purpose more accurately. The default value has also been updated to "dall-e-2" to align with the new configuration option.
This commit ensures that the OpenAI configuration is up-to-date and allows users to specify the desired image generation model.
