Welcome to Android Central! Are you sure they're downloaded from the web? Could they be images associated with an app that you installed, that are inadvertently included in the Gallery? If an app has images stored in a folder that it uses for its various screens (like, for example, icons for buttons), then those images will show up in your Gallery app, since the system's Media Scanner scans all folders for any images to place in there. To get rid of them, you can find the folder that the app uses and add a blank file titled ".nomedia", which instructs the Media Scanner to ignore that folder. You can use most file manager apps (like ES File Explorer) to create this blank file.