Damiaan, below is a class I used in UPillarv8 package.

using Newtonsoft.Json.Linq;
using System;
using System.IO;
using System.Web;
using System.Xml;
using System.Xml.Linq;
using Umbraco.Core;
using Umbraco.Core.Composing;
using Umbraco.Core.Models;
using Umbraco.Core.PackageActions;
using Umbraco.Core.Services;

namespace Kacebee.ClassLibrary.PackageActions
{
    public class AddMediaHandler : IPackageAction
    {
        IMediaService _mediaService = Current.Services.MediaService;

        public string Alias()
        {
            return "AddMediaHandler";
        }

        public bool Execute(string packageName, XElement xmlData)
        {
            var contentService = Current.Services.ContentService;
            var mediaTypeService = Current.Services.MediaTypeService;
            var mediaService = Current.Services.MediaService;
            var dataTypeService = Current.Services.DataTypeService;
            var fileService = Current.Services.FileService;

            try
            {
                Current.Logger.Info(System.Reflection.MethodBase.GetCurrentMethod().GetType(), "Executing CreateMediaHandler");
                return AddMediaItems();
            }
            catch (Exception ex)
            {
                Current.Logger.Error(System.Reflection.MethodBase.GetCurrentMethod().GetType(), "INSTALL Package Error", ex);
                return false;
            }
        }

        public bool AddMediaItems()
        {
            var contentService = Current.Services.ContentService;
            var mediaTypeService = Current.Services.MediaTypeService;
            var mediaService = Current.Services.MediaService;
            var dataTypeService = Current.Services.DataTypeService;
            var fileService = Current.Services.FileService;
            var contentTypeBaseServiceProvider = Current.Services.ContentTypeBaseServices;

            try
            {
                int parentId = -1;
                IMedia profileImage = AddMediaItem(parentId, "Profile.png", contentTypeBaseServiceProvider);

                Guid homeId = new Guid("777f5bc3-8f24-4dcb-b727-3b2e4473927e");
                IContent homePage = contentService.GetById(homeId);

                homePage.SetValue("siteLogo", profileImage.GetUdi().ToString());
                contentService.SaveAndPublish(homePage);

                if (homePage != null)
                {
                    contentService.SaveAndPublishBranch(homePage, true);
                }

                Current.Logger.Info(System.Reflection.MethodBase.GetCurrentMethod().GetType(), "Add media items");
                return true;
            }
            catch (Exception ex)
            {
                Current.Logger.Error(System.Reflection.MethodBase.GetCurrentMethod().GetType(), "Error when adding media items.", ex);
                return false;
            }
        }

        private IMedia AddMediaItem(int parentId, string fileName, IContentTypeBaseServiceProvider contentTypeBaseServiceProvider)
        {
            IMedia newFile = _mediaService.CreateMedia(fileName, parentId, "Image");
            try
            {
                string filePath = HttpContext.Current.Server.MapPath("~/img/" + fileName);
                using (FileStream stream = System.IO.File.Open(filePath, FileMode.Open))
                {
                    newFile.SetValue(contentTypeBaseServiceProvider, "umbracoFile", fileName, stream);
                }
                _mediaService.Save(newFile);
                Current.Logger.Info(System.Reflection.MethodBase.GetCurrentMethod().GetType(), "Add image");
                return newFile;
            }
            catch (Exception ex)
            {
                Current.Logger.Error(System.Reflection.MethodBase.GetCurrentMethod().GetType(), "Error when adding image", ex);
                return newFile;
            }
        }

        public XmlNode SampleXml()
        {
            const string sample = "<Action runat=\"install\" undo=\"true\" alias=\"AddMediaHandler\"></Action>";
            return ParseStringToXmlNode(sample);
        }

        private static XmlNode ParseStringToXmlNode(string value)
        {
            var xmlDocument = new XmlDocument();
            var xmlNode = AddTextNode(xmlDocument, "error", "");

            try
            {
                xmlDocument.LoadXml(value);
                return xmlDocument.SelectSingleNode(".");
            }
            catch
            {
                return xmlNode;
            }
        }

        private static XmlNode AddTextNode(XmlDocument xmlDocument, string name, string value)
        {
            var node = xmlDocument.CreateNode(XmlNodeType.Element, name, "");
            node.AppendChild(xmlDocument.CreateTextNode(value));
            return node;
        }

        public bool Undo(string packageName, XElement xmlData)
        {
            return true;
        }
    }
}