Background images on Strikingly are meant to conform to a variety of screen sizes, so parts of the background may be cropped depending on the screen size. (If viewing on a monitor, the top and bottom of the image may be cropped. If viewing on a phone in portrait mode, the left and right sides of the image may be cropped.)
For a more optimized background setting, an image dimension of 1600 x 900 px should do nicely.
Keep this in mind when you upload background images with content in them (faces, text, etc). If your background images contain content, it's best to keep this content in the center of the image, and add make sure there is enough space on the edges so cropping won't cut off your content. Another method is to upload the content as an image on top of the background, but this is only doable with certain sections.
It's also possible to alter the positioning/scaling settings of your background image.
If you need to display the entire image, try setting the scale option to "contain."
If you want the entire section to be covered by the image all the time, try "cover."
In case you need assistance, shoot us a line at firstname.lastname@example.org or chat with us.