I tried many different photo programs to edit the pictures of my illustrations that I post on Instagram until I found one that blew all the others away. Snapseed is free (!) and it allows you to do many semi-complex editing tasks on your phone, such as lighten a tiny section of your image or crisp-ify the edges of your drawing to make it stand out more. Often photos of sketchbooks mean that the images look dull, dark and not very contrasty when you take a phone picture (see above image on the left).
To download a printable pdf of tips that I put together to help you enhance your Instagram pics, click below:
Snapseed will make your illustrations look awesome on Instagram