You can make your images in anything that you're familiar with. The only requirement is that they are 256 x 80 pixels or 128 x 40 pixels, as per the jam rules. They must be bit-mapped graphics, not vector graphics. Adventuron supports images in gif and png formats, but try to limit the number of colours in the palette to keep the image size small. 8-bit png works well.
Hi Frosti,
Welcome to the jam.
Yes, you can use a web-browser based pixel editor like piskel.
In the Links at the bottom of the Adventuron documentation there are a couple of links to web based pixel editors (other than piskel).
Save all your location graphics as PNG files, and make sure your palette is 255 colours or less (palette recommendation on the jam page, but you can choose your own too).
Save them to your downloads folder on your chromebook, then you can import them into Adventuron via the IMPORT menu (please be sure to complete Tutorial A to show you how to do this).
"8-bit" is kind of a codeword for old-style computer games or computer art (depending on the context), generally speaking, the era of 1978 to 1988. These computers had a lot of natural limitations that do not exist today.
In the context of Adventuron "8-bit art" means graphics in a semi-limited style. As Garry said, the graphics should be in 256 x 80 resolution (it's important that the graphics are much wider than taller to leave space for text on the screen).
Chris