Since, like you said you are not an artist I would base your design only on the given APIs, Android and iOS ones. Like the user plainclothes told you be careful on the direction you are heading. If what you want it so build a game having the same design on both platforms is not an issue at all since it's usually the pattern. But if you are dealing with an application I wouldn't do it. I would instead try to follow the design guidelines of each OS. You can find the Android Design Guidelines here: http://developer.android.com/design/index.html
But if you still want to create graphics for your applications there are other problems you will face, true for both platforms but specially relevant for Android. I'm talking about dealing with multi-resolution and density screens. In order for your graphics to scale properly you will need to have different sets of graphics for different densities. On iOS this is typically two sets of graphics for each device type (iPhone, iPad, Retina and non-Retina, but I don't have much experience there so I can't actually tell you if this ir right). In terms of Android things get a bit "ouf of hand". You really need to have several resolutions of your graphic to cover all, or most of the different available devices. You can get a really good explanation here: http://developer.android.com/guide/practices/screens_support.html
To conclude, follow the design guidelines but in the last case just use the built-in design buttons or design your own using XML (for Android) for example.