Accessing photos through gmail or other uploading site.

The pictures that I have taken using my iPhone are all visible on my Mac Air through the photos icon. However, recently, when I am in gmail and try to access my pictures to upload through an attachment, hardly any of the pictures are visible. This is also the case when I try to upload photos to a site like Walmart to have prints made. I have done this successfully in the past many times so I'm not sure what the difference is now. Thank you!