The optimal file format for you to use on your Photo Display is either '.jpeg' (or any of its extensions .jpg, .jpe and .jiff) or '.png'.
If you try to use other file formats such as Bitmap (.bmp), GIF (.gif) TIFF (.tiff) and so on, a message will pop up telling you that the upload failed. (Shown below)
Your image may appear that it is in a .jpg format but actually it was just renamed into a .jpg file. When you open it, your PC would recognize it as a .jpg file but when you put it inside a Content you will encounter an error message saying you got the wrong file format.
In the sample above, you can see a bitmap (.bmp) image being converted into a .jpg format. And once it is downloaded, your PC recognizes it as a .jpg file.
Once you try to include it in your Folder, a message will appear on the screen saying the upload failed. This happened because the photo is still in bitmap type even after saving it as .jpg in your PC.
Note: It is really important to determine whether the photos that we are going to include in our Folder are .jpeg/.jpg or .png formats and not just altered photos to avoid unexpected errors in uploading your photos to your Folder.