Depending on which phones you need to support, you may encounter some headaches.
A few things you need to consider:
You need to consider a huge variety of screen sizes.
Larger screens will be able to display large images, but smaller screens will require smaller versions of the images.
Yes, as you suspect, not all browsers support all image formats.
Need to support touch screen and devices without touch screen? If you do this, you will need to consider the size of the elements you need to touch.
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