In the default template there is a slider at the top of the main page advertising features of Abantecart.
I can't figure out how to edit this to put my own stuff there.
I looked at Banner Manager but can't see any option there to edit the slider to change the images and text.
You can to edit banner slider and settings in 2 places:
Design -> Banner Manager (locate and edit all Main Banners)
Banners are in HTML, so you can edit text and path to another image
You can use any image size that fits your requirements. Current sizes are 600x300
Design -> Blocks -> Main Page Banner Slider.
Here you can control sequence of banners and add/remove them