Place the aspx file inside your module directory and adjuth your pathes that it searches your images at ~/desktopmodules/yourmodule instead of ~/custompages.
Then you can treat the aspx inside the dnn file like all other files.

More comfortable :

        //  /DesktopModules/CustomModule/ViewCustomModule.ascx.cs
      
        public static string ResolveDesktopModuleUrl(DotNetNuke.Entities.Modules.PortalModuleBase portalModule, string relativePath)
        {
            return ResolveDesktopModuleUrl(portalModule, relativePath, null);
        }

        public static string ResolveDesktopModuleUrl(DotNetNuke.Entities.Modules.PortalModuleBase portalModule, string relativePath, string fileName)
        {
            if (portalModule == null) throw new ArgumentNullException("portalModule");
            return ResolveDesktopModuleUrl(portalModule.ModuleId, portalModule.TabId, relativePath, fileName);
        }

        public static string ResolveDesktopModuleUrl(int moduleId, int tabId, string relativePath, string fileName)
        {
            const string DESKTOP_MODULES_FOLDER_NAME = "/DesktopModules/";

            DotNetNuke.Entities.Modules.ModuleController moduleController = new DotNetNuke.Entities.Modules.ModuleController();
            DotNetNuke.Entities.Modules.DesktopModuleController desktopModuleController = new DotNetNuke.Entities.Modules.DesktopModuleController();

            DotNetNuke.Entities.Modules.ModuleInfo moduleInfo = moduleController.GetModule(moduleId, tabId);
            DotNetNuke.Entities.Modules.DesktopModuleInfo desktopModuleInfo = desktopModuleController.GetDesktopModule(moduleInfo.DesktopModuleID);

            if (string.IsNullOrEmpty(relativePath) == false)
            {
                if (relativePath.StartsWith("/") == false) relativePath = "/" + relativePath;

                if (string.IsNullOrEmpty(fileName) == false)
                {
                    if (relativePath.EndsWith("/") && fileName.StartsWith("/"))
                    {
                        relativePath = relativePath.Remove(relativePath.Length - 1);
                    }
                    else if (relativePath.EndsWith("/") == false && fileName.StartsWith("/") == false)
                    {
                        relativePath += "/";
                    }

                    return Globals.ResolveUrl(DotNetNuke.Common.Globals.ApplicationPath + DESKTOP_MODULES_FOLDER_NAME + desktopModuleInfo.FolderName + relativePath + fileName);
                }

                return Globals.ResolveUrl(DotNetNuke.Common.Globals.ApplicationPath + DESKTOP_MODULES_FOLDER_NAME + desktopModuleInfo.FolderName + relativePath);

            }
            else
            {
                if (string.IsNullOrEmpty(fileName) == false)
                {
                    if (fileName.StartsWith("/") == false)
                    {
                        fileName = "/" + fileName;
                    }

                    return Globals.ResolveUrl(DotNetNuke.Common.Globals.ApplicationPath + DESKTOP_MODULES_FOLDER_NAME + desktopModuleInfo.FolderName + fileName);
                }
            }

            return Globals.ResolveUrl(DotNetNuke.Common.Globals.ApplicationPath + DESKTOP_MODULES_FOLDER_NAME + desktopModuleInfo.FolderName);

        }

       Usage: 

        string pageUrl = ResolveDesktopModuleUrl(this,"SomePage.aspx");
       
        resolved to /DesktopModules/CustomModule/SomePage.aspx