Images are going to be dynamically resized based on the display. Basically, this means that there are no one-size-fits-all guidelines. You'll find our recommendations below, but for the best results, you should test viewing the polls on the actual screen where they will be displayed and adjust accordingly.
Ideally, the images will all be the same size/aspect ratio as the display screen. This will ensure that they 'fill' the response containers and the chart equally. We recommend minimum 800px width and height (we'll keep the aspect ratio regardless of file size).
We recommend a minimum size of 800px. That size is big enough that it will look good on a retina display with 2x pixel density while ensuring that the image is big enough for the average phone/tablet. Better yet, go larger. If you want to make sure it's going to look good on a 1080 display, go for something >1800px to match the real estate the poll will use when displayed in fullscreen mode. But, of course, you have to balance an image that large with the upload file limitation of 5MB.
We recommend a logo that is ~250x250 pixels, or 2x the display area.
Dimensions and resolution should ideally match the screen resolution that you are presenting on, but there is a file size limit of 5MB.