Since there are a few of you having problems I thought I'd figure out what's going on and write up a quick tutorial.
Unless you are using tapatalk, this is how you would do it.
First off, make sure you are using the full theme.
1. Click Chose files and pick the file you want to upload.
2. Click "Attach this file"
Repeat step 1 and 2 repeatedly until you have all the pictures you want attached.