You can upload your photos as .jpeg or .jpg.
These formats are also valid with capital lettering. Jpg files ensure faster loading while containing a high quality and therefore the visitors of your ads will have a great user experience.
If your photos have a different format than the ones mentioned above, please use a standard graphic program to convert them. This can usually be done by either using the export function or simply use save as and choose a different format from the drop down menu.