It depends. Are you talking about a ‘Slide Background’ image (see the ‘Slide Background’ sections on the edit page), or are you talking about images you insert into a slide using the slide editor box (these are foreground or layer images)?
With slide background images, you can use the options within the ‘Background Size’ drop-down to stretch the background image to cover (or be contained) within the slide size – you can experiment with this setting.
With foreground (layered) slide images, look at the ‘Insert Media’ box which appears when you click the ‘Add Media’ button. If you look under ‘ATTACHMENT DISPLAY SETTINGS’ (on the right-hand side), you should see under the ‘Size’ setting the options: ‘Thumbnail’, ‘Medium’, ‘Large’ and ‘Full Size’. Unfortunately if your image is tiny to begin with (120px by 60px) you wont see these options, but to stretch the image this small will make it lose resolution (become blurry) anyway, and you should probably source bigger images.
