Do the ad widgets really need differently sized images created for each ad, or will they automatically resize to other screen sizes from the responsiveness of the template?
You don't necessarily need images with different sizes, as images will fit to screen width automatically. However, it would be better to have different images optimized for different screen sizes (you can check the ads on Bone's demo). If you're using only 1 image, you might just put them in a text widget, that will save you from copying/pasting for different ad code fields.
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OK, thanks for the advice :)