As of AC v1.73.0, changes made to character portrait textures (using the "Character: Change portrait" Action) are no longer saved automatically. To save such changes, attach the RememberPortrait script to the character:

using UnityEngine;
#if AddressableIsPresent
using System.Collections;
using UnityEngine.ResourceManagement.AsyncOperations;
using UnityEngine.AddressableAssets;
#endif

namespace AC
{

	[AddComponentMenu ("Adventure Creator/Save system/Remember Portrait")]
	public class RememberPortrait : Remember
	{

		private Char character;

		public override string SaveData ()
		{
			PortraitData portraitData = new PortraitData ();
			portraitData.objectID = constantID;
			portraitData.savePrevented = savePrevented;
			if (Character)
			{
				portraitData.portraitGraphic = AssetLoader.GetAssetInstanceID (Character.portraitIcon.texture);
			}
			return Serializer.SaveScriptData<NameData> (portraitData);
		}

		public override void LoadData (string stringData)
		{
			PortraitData data = Serializer.LoadScriptData<PortraitData> (stringData);
			if (data == null) return;
			SavePrevented = data.savePrevented; if (savePrevented) return;
			if (Character && !string.IsNullOrEmpty (data.portraitGraphic))
			{
				#if AddressableIsPresent
				if (KickStarter.settingsManager.saveAssetReferencesWithAddressables)
				{
					StartCoroutine (LoadDataFromAddressables (data));
				}
				else
				#endif
				{
					Character.portraitIcon.ReplaceTexture (AssetLoader.RetrieveAsset (Character.portraitIcon.texture, data.portraitGraphic));
				}
			}
		}

		#if AddressableIsPresent

		private IEnumerator LoadDataFromAddressables (PortraitData data)
		{
			AsyncOperationHandle<Texture> textureHandle = Addressables.LoadAssetAsync<Texture> (data.portraitGraphic);
			yield return textureHandle;
			if (textureHandle.Status == AsyncOperationStatus.Succeeded)
			{
				Character.portraitIcon.ReplaceTexture (textureHandle.Result);
			}
			Addressables.Release (textureHandle);
		}

		#endif

		private Char Character
		{
			get
			{
				if (character == null) character = GetComponent <Char>();
				return character;
			}
		}

	}

	[System.Serializable]
	public class PortraitData : RememberData
	{
		public string portraitGraphic;
		public PortraitData () { }
	}

}
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