You can attach files and images on your entered expenses and incomes. Let's suppose you receive your monthly salary, you can take a picture of your paystub and attache this picture to the income transaction you entered when you received it.
On Mobills main screen, touch the 'Transactions' icon, as in the image below and then choose an expense or income that you wish to attach a file to:
When editing the transaction to attach the files you want to, touch one of the 'Attach' icons, as in the image below. By touching one of the icons, you can either take a picture ('Camera' icon) or go to your gallery and add a picture that already exists ('Frame' icon).
After selecting your file, just save all the changes! :D
Be aware! This tutorial can be used to attach files for both incomes and expenses.