As a followup to my earlier question on a custom UI Button with image, I was able to create round custom buttons with images embedded to it. I’m not a good designer at all and wanted to get expert advises on the following:
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Are round buttons better looking and won’t the users confuse or wrongly press the Home button instead of my app’s round buttons when using the app?
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Are these too big and what would be the optimum size I can go with? The app I’m building is both for iPhone (include iPhone 5) and iPad including mini.
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Will the buttons look better with just plain text and without the embedded images?
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The background color of the View is black and would like to know what would be the button’s optimum color and size I can go with?
Thanks.

The user will generally be able to distinguish between their devices home button (which they know quite well) and a custom button on the devices screen. Round buttons can be an effective way to present a touch element that has a single correct point of touch. Look at Path for a great example of round buttons. From your example it looks like you are trying to reimplement UITabBar. Please consider using the built in implementation as it still provides plenty of UI customization and is additionally quite a recognizable element for the average user.
The buttons should meet the minimum Apple Human Interface Guidelines for mobile applications. You may consider making the actual button images smaller, but surrounding them with transparent padding to increase the hit target so that users will have an easy time actually hitting your buttons.
Generally I find that buttons with images are preferable to text, esp. when the images accurately convey meaning. The image button will make the user more inquisitive about your application, plus once they learn the function of a button, they don’t need the text anymore.
Without seeing more of the application I would not be able to give you more insight about actual design or color choice. Typically, black interfaces tend to provide negative experiences for the user. You will notice that most of the popular apps on the web feature lots of white or light gray, and plenty of space between elements. Even Apple rarely uses a true black element.