Each image or file uploaded cannot be more than 30MB. While images with dimensions up to 5000 x 5000 pixels can be uploaded, their on-screen display will not exceed 1920 x 1080 pixels. Images can be uploaded within Website Builder (more info).
Note: If you exceed your account's allotted bandwidth, your account will be automatically upgraded to the bandwidth level needed to handle your extra usage by 5GB, 10GB or 15GB. You will be automatically charged for that extra amount. Your account will remain at that more expensive, higher bandwidth level until you manually reset it to the previous lower level.
If you expect to temporarily need more bandwidth for a month or more, you can prepay to increase it for that period. For more on changing your Website Builder's bandwidth allotment, see Upgrading or downgrading your plan.