You can see your picture format by going to "Details". On the iPhone, I believe it's the 3 or 4 horizontal bars near the top of the picture. I have an Android phone, so I'm not as keen on the iOS, but there is definitely a way to view the details of the pic, such as the time stamp (date it was created/last modified), file size, resolution, and format.
I don't know that GIMP is available in the app store, but it won't hurt to take a look. GIMP is more powerful than what you would need to change a simple file format though. You can edit the file type very simply by changing the characters that proceed the "." after the file name.
Ex: Change "avatar.png" to "avatar.jpg".
