Need help posting pics to for sale ad

I will give a quick demonstration on how I do it with my phone i have a android so it may be different than iPhone I hope this helps first click the upload photos in the option bar, then select files, on mine i can select from different folders like my gallery or downloads or images that are saved or I can select from my recent files. For this im gonna click gallery then I click on camera which are the photos iv take with my camera. Then just click the picture I want. If its a download or something from somewhere else I would have to go to my files and find it or just screenshot it that automatically saves it in the gallery. I hope this helps if not feel free to ask I will try my best to help