The image on the mini-poster is intended to be illustrative. It is not an image that it is intended the viewer will look at in detail, because if they want more detail about your work then they can scan the QR code on your mini-poster and open and explore your full ePoster/stack.
The ideal dimensions of the image for your mini-poster are 1200 x 280 pixels or any ratio 30:7.
When you add an image you can also choose to crop it to these dimensions or zoom in or out to make the key parts of the image fit into these dimensions. This FAQ answer provides more information about cropping and adjusting the size of the image within Learning Toolbox.