For a twitter post you may provide text, URL and a picture as Texture2D. See the method description below:

TwitterPost(string text, string url = null, Texture2D texture = null)

You may also subscribe to a competition event to find out if a user has posted to Twitter. See the code snippet below:

Texture2D tex = GetTextureForPost();
IOSSocialManager.Instance.TwitterPost("My app Screenshot","",  tex);
IOSSocialManager.OnTwitterPostResult += HandleOnTwitterPostResult;

void HandleOnTwitterPostResult (ISN_Result res){
	if(res.IsSucceeded) {
		IOSNativePopUpManager.showMessage("Posting example", "Post Success!");
	} else {
		IOSNativePopUpManager.showMessage("Posting example", "Post Failed :(");

You can also post gif image to the twitter feed providing gif image device path. Using the following method of the IOSSocialManager

public void TwitterPostGif(string text, string url)

Please, note that posting process will run in background and may take a while. The same OnTwitterPostResult action will be fired once a post request is completed.