Your tiger looks awesome!
For Apple devices, getting the file size is a little complicated. You can download an app that will help you. http://www.wikihow.com/Find-the-File-Size-of-an-iOS-Photo
While I’m not in the publishing or illustration industry, I have spent a lot of time in kids books recently, and all kinds of art is acceptable there. It comes down to the age group you are targeting. I’m sure the tiger would cover many age groups.
Lots of kid books tell a story or support a story with their illustrations – and those have been the most successful in my observations. 3 that seem to be in my current rotation – “The Interrupting Chicken” – Stein,”The Magic Pot” – Coombs, “The Life of Fred” series – Schmidt. If you can tell a story in your style, then i’d say you are ready to go!