App Designed for my php website

I own a website [login to view URL] It is an Ebay type website but it is free for everyone to use. It is a combination of auctions and classifieds. 98% are classifieds! I need customers to be able to upload items to the site directly from their phone. Iphones, smart phones etc. Listings are FREE but clients are not uploading their items. Local Facebook selling "groups" get loads of items because people can upload the items from their phones in seconds and I cant compete. EG, Clients register on my site but then they don't end up loading their items as they get confused. Over 1300 people have registered so that is ok. I have 400 listing but should have many more. I am getting loads of feedback and this is what is happening... 1. People are taking pictures on their phones, 2. They then have trouble uploading them to their desktop. (Don't know how to!) 3. They are too big and they cant resize them. My site cant resize large pictures. (This happens every day) 4. They then find adding a listing complicated. I want an app designed where they can register on the site, and add items to the site quickly and easily using their smart phone. Is this possible and how much would it cost? Thanks, Brian.