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On the meta posts that collect community ads (example), I see ads in two sizes, 300x250px and 600x500px. (Everything renders the same when ads are actually used on the sites, as you would expect.) The requirements for ads include:

The image that you create must be 300 x 250 pixels, or double that if high DPI.

I assume "high DPI" is a property of the software used to create the ads, or perhaps some "image export" property. I'm not a graphics person; the ads I've made have usually started out as photos or similar art that I crop and scale to 300x250 using low-end software (Paintbrush on Mac, Paint on Windows, that kind of thing). Should I be making them double size? I haven't because nothing in my software says anything about "high DPI" so I assume it doesn't apply to me. In a few months we'll be doing the next round of ads on my sites, and if I should be doing something different to make better images, I'd like to know.

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You just need to have the image on 600 x 500 pixels.

The "high DPI" refers to the display screen, not the image. Dara Skolnick explains this matter better,

Hey, what about retina displays?

Glad you asked! Retina displays, also known as "high DPI" displays, are awesome in that they cram more pixels into their screens than standard displays. This means that each individual pixel is even smaller so you get a much sharper image than on a standard screen. So doesn't image DPI matter now? Doesn't it?

Nope. All you really need to know is that if you're optimizing for a retina display that's squishing double the number of pixels into the same area as a non-retina display would, you need to double the pixels in your image so it still appears crisp. So, a logo that's 300px across normally should be saved at 600px. Then, when that image is presented at 300px as originally intended, it'll look nice and crisp on retina screens since the image has double the number of pixels and so does the screen. The DPI/PPI setting in Photoshop is still irrelevant because you're not printing it out.

If in doubt, remember that on the web, it's all about the pixels. [...].

However, if possible, don't just upscale the 300 x 250 pixels into 600 x 500 pixels because usually, it won't look better (i.e. blurry, defeats the purpose of trying to create a sharper image). Instead, design the ads from the scratch as if it's for 600 x 500 pixels originally.

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