To upload images into the application I would recommend using the image uploader widget. This is included in Mendix, so no need to download anything additional. If you double click on the image uploader on the page, you go to the settings of the widget. here you can select the allowed extensions (see screenshot). In this case you want to fill in: jpg; jpeg; png; When uploading a file users will now only see files with these extensions.
Hope this helps!